Index: tests/natools-s_expressions-encodings-tests.adb
==================================================================
--- tests/natools-s_expressions-encodings-tests.adb
+++ tests/natools-s_expressions-encodings-tests.adb
@@ -22,10 +22,11 @@
 
    procedure All_Tests (Report : in out NT.Reporter'Class) is
    begin
       Hexadecimal_Test (Report);
       Base64_Test (Report);
+      User_Base64_Test (Report);
    end All_Tests;
 
 
    procedure Hexadecimal_Test (Report : in out NT.Reporter'Class) is
       All_Octets : Atom (1 .. 256);
@@ -252,6 +253,60 @@
               & Ada.Exceptions.Exception_Name (Error)
               & " has been raised");
       end;
    end Base64_Test;
 
+
+   procedure User_Base64_Test (Report : in out NT.Reporter'Class) is
+      Test : NT.Test := Report.Item ("Base-64 with user-defined charset");
+   begin
+      declare
+         Digit_62 : constant Octet := Character'Pos ('-');
+         Digit_63 : constant Octet := Character'Pos ('_');
+            --  Charset for Base-64 URI (RFC 4648)
+
+         Padding : constant Octet := Character'Pos ('|');
+
+         Source : constant Atom
+           := (4#0000#, 4#0100#, 4#2003#,   4#0100#, 4#1101#, 4#2013#,
+               4#0200#, 4#2102#, 4#2023#,   4#0300#, 4#3103#, 4#2033#,
+               4#1001#, 4#0110#, 4#2103#,   4#1101#, 4#1111#, 4#2113#,
+               4#1201#, 4#2112#, 4#2123#,   4#1301#, 4#3113#, 4#2133#,
+               4#2002#, 4#0120#, 4#2203#,   4#2102#, 4#1121#, 4#2213#,
+               4#2202#, 4#2122#, 4#2223#,   4#2302#, 4#3123#, 4#2233#,
+               4#3003#, 4#0130#, 4#2303#,   4#3103#, 4#1131#, 4#2313#,
+               4#3203#, 4#2132#, 4#2323#,   4#3303#, 4#3133#, 4#2333#,
+               16#42#);
+         Expected : constant Atom
+           := (65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77,
+               78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90,
+               97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109,
+               110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122,
+               48, 49, 50, 51, 52, 53, 54, 55, 56, 57, Digit_62, Digit_63,
+               81, 103, Padding, Padding);
+         Encoded_Short : constant Atom
+           := Encode_Base64 (Source, Digit_62, Digit_63);
+         Encoded_Long : constant Atom
+           := Encode_Base64 (Source, Digit_62, Digit_63, Padding);
+      begin
+         Test_Tools.Test_Atom (Test, Expected, Encoded_Long);
+
+         Test_Tools.Test_Atom
+           (Test,
+            Expected (Expected'First .. Expected'Last - 2),
+            Encoded_Short);
+
+         Test_Tools.Test_Atom
+           (Test,
+            Source,
+            Decode_Base64 (Encoded_Long, Digit_62, Digit_63));
+
+         Test_Tools.Test_Atom
+           (Test,
+            Source,
+            Decode_Base64 (Encoded_Short, Digit_62, Digit_63));
+      end;
+   exception
+      when Error : others => Test.Report_Exception (Error);
+   end User_Base64_Test;
+
 end Natools.S_Expressions.Encodings.Tests;

Index: tests/natools-s_expressions-encodings-tests.ads
==================================================================
--- tests/natools-s_expressions-encodings-tests.ads
+++ tests/natools-s_expressions-encodings-tests.ads
@@ -29,8 +29,9 @@
    procedure All_Tests (Report : in out NT.Reporter'Class);
       --  Run all available tests
 
    procedure Hexadecimal_Test (Report : in out NT.Reporter'Class);
    procedure Base64_Test (Report : in out NT.Reporter'Class);
+   procedure User_Base64_Test (Report : in out NT.Reporter'Class);
       --  Run test suite for each encoding type
 
 end Natools.S_Expressions.Encodings.Tests;