diff --git a/source/XeroApi.ConsoleTests/Program.cs b/source/XeroApi.ConsoleTests/Program.cs
index a2110a4..a777848 100644
--- a/source/XeroApi.ConsoleTests/Program.cs
+++ b/source/XeroApi.ConsoleTests/Program.cs
@@ -3,7 +3,8 @@
 using System.IO;
 using System.Linq;
 using DevDefined.OAuth.Utility;
-using XeroApi.Model;
+using XeroApi.Model;
+using XeroApi.Model.Constants;
 using XeroApi.Model.Reporting;
 
 namespace XeroApi.ConsoleApp
@@ -61,6 +62,7 @@ static void ExerciseOrganisation(Repository repository)
             TestFindingTrackingCategories(repository);
             TestFindingItemsUsingLinqSyntax(repository, organisation);
             TestCreatingInvoiceWithValidationErrors(repository);
+            TestCreatingInvoiceWithValuesFromStringConstants(repository);
             TestAttachmentFromByteArray(repository);
             TestAttachmentsAgainstPurchaseInvoice(repository);
             
@@ -359,7 +361,7 @@ private static void TestFindingInvoicesByContactName(Repository repository, Cont
         private static void TestCreatingInvoiceWithValidationErrors(Repository repository)
         {
             // Try and create an invoice - but using incorrect data. This should hopefully be rejected by the Xero API
-            Invoice invoiceToCreate = new Invoice
+            var invoiceToCreate = new Invoice
                 {
                     Contact = new Contact
                         {
@@ -392,6 +394,43 @@ private static void TestCreatingInvoiceWithValidationErrors(Repository repositor
             }
         }
 
+        private static void TestCreatingInvoiceWithValuesFromStringConstants(Repository repository)
+        {
+            var contact = repository.Contacts.FirstOrDefault() ?? new Contact
+                {
+                    Name = "Test Contact",
+                };
+
+            // Try and create an invoice with correct data. But using values that would be passed in as strings from helper class
+            var invoiceToCreate = new Invoice
+            {
+                Type = InvoiceTypes.AccountsReceivable,
+                Status = InvoiceStatusCodes.Draft,
+                Contact = contact,
+                Date = DateTime.Today,
+                DueDate = DateTime.Today.AddDays(30),
+                LineAmountTypes = LineAmountType.NoTax,
+                LineItems = new LineItems
+                        {
+                            new LineItem
+                                {
+                                    Description = "New Item",
+                                    Quantity = 1,
+                                    UnitAmount = 300,
+                                    TaxType = LineItemTaxTypes.NewZealand.ZeroRated
+                                }
+                        }
+            };
+
+            Console.WriteLine("Creating an invoice that should cause a validation error...");
+            var createdInvoice = repository.Create(invoiceToCreate);
+
+            if (createdInvoice.ValidationStatus == ValidationStatus.OK)
+            {
+                Console.WriteLine("Invoice created successfully");
+            }
+        }
+
         private static void TestFindingItemsUsingLinqSyntax(Repository repository, Organisation organisation)
         {
             // Try the linq syntax to select items with sales details..
diff --git a/source/XeroApi/Model/Constants/InvoiceStatusCodes.cs b/source/XeroApi/Model/Constants/InvoiceStatusCodes.cs
new file mode 100644
index 0000000..fd369f5
--- /dev/null
+++ b/source/XeroApi/Model/Constants/InvoiceStatusCodes.cs
@@ -0,0 +1,30 @@
+namespace XeroApi.Model.Constants
+{
+    public class InvoiceStatusCodes
+    {
+        public static string Draft
+        {
+            get { return "DRAFT"; }
+        }
+        public static string Submitted
+        {
+            get { return "SUBMITTED"; }
+        }
+        public static string Deleted
+        {
+            get { return "DELETED"; }
+        }
+        public static string Authorised
+        {
+            get { return "AUTHORISED"; }
+        }
+        public static string Paid
+        {
+            get { return "PAID"; }
+        }
+        public static string Voided
+        {
+            get { return "VOIDED"; }
+        }
+    }
+}
diff --git a/source/XeroApi/Model/Constants/InvoiceTypes.cs b/source/XeroApi/Model/Constants/InvoiceTypes.cs
new file mode 100644
index 0000000..b437810
--- /dev/null
+++ b/source/XeroApi/Model/Constants/InvoiceTypes.cs
@@ -0,0 +1,15 @@
+namespace XeroApi.Model.Constants
+{
+    public class InvoiceTypes
+    {
+        public static string AccountsPayable
+        {
+            get { return "ACCPAY"; }
+        }
+
+        public static string AccountsReceivable
+        {
+            get { return "ACCREC"; }
+        }
+    }
+}
diff --git a/source/XeroApi/Model/Constants/LineItemTaxTypes.cs b/source/XeroApi/Model/Constants/LineItemTaxTypes.cs
new file mode 100644
index 0000000..ef28e8f
--- /dev/null
+++ b/source/XeroApi/Model/Constants/LineItemTaxTypes.cs
@@ -0,0 +1,201 @@
+namespace XeroApi.Model.Constants
+{
+    public class LineItemTaxTypes
+    {
+        public class NewZealand
+        {
+            public static string Input
+            {
+                get { return "INPUT"; }
+            }
+            public static string Input2
+            {
+                get { return "INPUT2"; }
+            }
+            public static string None
+            {
+                get { return "NONE"; }
+            }
+            public static string ZeroRated
+            {
+                get { return "ZERORATED"; }
+            }
+            public static string Output
+            {
+                get { return "OUTPUT"; }
+            }
+            public static string Output2
+            {
+                get { return "OUTPUT2"; }
+            }
+        }
+
+        public class UnitedKingdom
+        {
+            public static string Capexinput
+            {
+                get { return "CAPEXINPUT"; }
+            }
+            public static string Capexoutput
+            {
+                get { return "CAPEXOUTPUT"; }
+            }
+            public static string Capexinput2
+            {
+                get { return "CAPEXINPUT2"; }
+            }
+            public static string Capexoutput2
+            {
+                get { return "CAPEXOUTPUT2"; }
+            }
+            public static string Eczrinput
+            {
+                get { return "ECZRINPUT"; }
+            }
+            public static string Eczroutput
+            {
+                get { return "ECZROUTPUT"; }
+            }
+            public static string Exemptinput
+            {
+                get { return "EXEMPTINPUT"; }
+            }
+            public static string Exemptoutput
+            {
+                get { return "EXEMPTOUTPUT"; }
+            }
+            public static string Input
+            {
+                get { return "INPUT"; }
+            }
+            public static string Input2
+            {
+                get { return "INPUT2"; }
+            }
+            public static string None
+            {
+                get { return "NONE"; }
+            }
+            public static string Output
+            {
+                get { return "OUTPUT"; }
+            }
+            public static string Output2
+            {
+                get { return "OUTPUT2"; }
+            }
+            public static string Rrinput
+            {
+                get { return "RRINPUT"; }
+            }
+            public static string Rroutput
+            {
+                get { return "RROUTPUT"; }
+            }
+            public static string Srinput
+            {
+                get { return "SRINPUT"; }
+            }
+            public static string Sroutput
+            {
+                get { return "SROUTPUT"; }
+            }
+            public static string ZeroRatedInput
+            {
+                get { return "ZERORATEDINPUT"; }
+            }
+            public static string ZeroRatedOutput
+            {
+                get { return "ZERORATEDOUTPUT"; }
+            }
+        }
+
+        public class Australia
+        {
+            public static string Output
+            {
+                get { return "OUTPUT"; }
+            }
+            public static string Input
+            {
+                get { return "INPUT"; }
+            }
+            public static string Capexinput
+            {
+                get { return "CAPEXINPUT"; }
+            }
+            public static string Exemptexport
+            {
+                get { return "EXEMPTEXPORT"; }
+            }
+            public static string Exemptexpenses
+            {
+                get { return "EXEMPTEXPENSES"; }
+            }
+            public static string Exemptcapital
+            {
+                get { return "EXEMPTCAPITAL"; }
+            }
+            public static string Exemptoutput
+            {
+                get { return "EXEMPTOUTPUT"; }
+            }
+            public static string Inputtaxed
+            {
+                get { return "INPUTTAXED"; }
+            }
+            public static string Basexcluded
+            {
+                get { return "BASEXCLUDED"; }
+            }
+            public static string Gstoncapimports
+            {
+                get { return "GSTONCAPIMPORTS"; }
+            }
+            public static string Gstonimports
+            {
+                get { return "GSTONIMPORTS"; }
+            }
+        }
+
+        public class Global
+        {
+            public static string Input
+            {
+                get { return "INPUT"; }
+            }
+            public static string None
+            {
+                get { return "NONE"; }
+            }
+            public static string Output
+            {
+                get { return "OUTPUT"; }
+            }
+            public static string Gstonimports
+            {
+                get { return "GSTONIMPORTS"; }
+            }
+        }
+
+        public class UnitedStates
+        {
+            public static string Input
+            {
+                get { return "INPUT"; }
+            }
+            public static string None
+            {
+                get { return "NONE"; }
+            }
+            public static string Output
+            {
+                get { return "OUTPUT"; }
+            }
+            public static string Gstonimports
+            {
+                get { return "GSTONIMPORTS"; }
+            }
+        }
+    }
+}
diff --git a/source/XeroApi/XeroApi.csproj b/source/XeroApi/XeroApi.csproj
index 3ba7606..313b379 100644
--- a/source/XeroApi/XeroApi.csproj
+++ b/source/XeroApi/XeroApi.csproj
@@ -66,6 +66,9 @@
     <Compile Include="Model\Attachment.cs" />
     <Compile Include="Model\BankTransaction.cs" />
     <Compile Include="Model\BrandingTheme.cs" />
+    <Compile Include="Model\Constants\InvoiceStatusCodes.cs" />
+    <Compile Include="Model\Constants\InvoiceTypes.cs" />
+    <Compile Include="Model\Constants\LineItemTaxTypes.cs" />
     <Compile Include="Model\ContactGroup.cs" />
     <Compile Include="Model\CreditNote.cs" />
     <Compile Include="Model\Currency.cs" />