diff --git a/Contrib/Makensisw/makensisw.cpp b/Contrib/Makensisw/makensisw.cpp
index 0ce10b0a..2edb6454 100644
--- a/Contrib/Makensisw/makensisw.cpp
+++ b/Contrib/Makensisw/makensisw.cpp
@@ -31,7 +31,11 @@
 #include "update.h"
 
 namespace MakensisAPI {
+#ifdef _WIN64
+  const TCHAR* SigintEventNameFmt = _T("makensis win32 sigint event %Iu");
+#else
   const TCHAR* SigintEventNameFmt = _T("makensis win32 sigint event %u");
+#endif
   const TCHAR* SigintEventNameLegacy = _T("makensis win32 signint event");
 }
 
diff --git a/Docs/src/history.but b/Docs/src/history.but
index 18e81833..c530edfa 100644
--- a/Docs/src/history.but
+++ b/Docs/src/history.but
@@ -6,6 +6,10 @@ Released on ?, 2014
 
 \S1{v3.0b1-cl} Changelog
 
+\S2{} Major Changes
+
+\b MakeNSIS WM_COPYDATA messages now use the QH_OUTPUTCHARSET encoding with CP_ACP as the default for compatibility with old IDEs.
+
 \S2{} Minor Changes
 
 \b Fixed POSIX !searchparse bug (\W{http://sf.net/p/nsis/patches/251}{patch #251})
diff --git a/Source/build.cpp b/Source/build.cpp
index 0902caba..9fcc25ff 100644
--- a/Source/build.cpp
+++ b/Source/build.cpp
@@ -70,7 +70,11 @@ bool isSimpleChar(TCHAR ch)
 } // end of anonymous namespace
 
 namespace MakensisAPI {
-  const TCHAR* SigintEventNameFmt = _T("makensis win32 sigint event %u"); // %u is the notify HWND, this is to make sure we abort the correct instance
+#ifdef _WIN64
+  const TCHAR* SigintEventNameFmt = _T("makensis win32 sigint event %Iu"); // %u is the notify HWND, this is to make sure we abort the correct instance
+#else
+  const TCHAR* SigintEventNameFmt = _T("makensis win32 sigint event %u");
+#endif
   const TCHAR* SigintEventNameLegacy = _T("makensis win32 signint event"); // "sigNint" typo is part of the API now and cannot be changed
 }
 
@@ -3456,7 +3460,20 @@ void CEXEBuild::notify(MakensisAPI::notify_e code, const TCHAR *data) const
 #ifdef _WIN32
   if (notify_hwnd)
   {
-    COPYDATASTRUCT cds = {(DWORD)code, (UINT32)(_tcslen(data)+1)*sizeof(TCHAR), (void *) data};
+    DWORD cb = (DWORD) (_tcslen(data)+1) * sizeof(TCHAR);
+#ifdef _UNICODE
+    extern NStreamEncoding g_outputenc;
+    extern void quit();
+    CharEncConv cec;
+    if (!g_outputenc.IsUTF16LE())
+    {
+      size_t cbConv;
+      if (!cec.Initialize(g_outputenc.GetCodepage(), -1) || !(data = (const TCHAR*) cec.Convert(data, cb, &cbConv)))
+        PrintColorFmtMsg_ERR(_T("conversion failed!\n")), quit(); // Cannot use ERROR_MSG() here!
+      cb = (DWORD) (cbConv + NStreamEncoding::GetCodeUnitSize(g_outputenc.GetCodepage())); // cbConv does not include the \0.
+    }
+#endif
+    COPYDATASTRUCT cds = {(DWORD) code, cb, (void*) data};
     SendMessage(notify_hwnd, WM_COPYDATA, 0, (LPARAM)&cds);
   }
 #endif
diff --git a/Source/build.h b/Source/build.h
index c4bbb90e..32ea887d 100644
--- a/Source/build.h
+++ b/Source/build.h
@@ -85,7 +85,7 @@ namespace MakensisAPI {
     QUERYHOST = WM_APP // QUERYHOST_e in wParam
   };
   enum QUERYHOST_e {
-    QH_OUTPUTCHARSET = 1 // return (wincodepage+1) or 0 for default
+    QH_OUTPUTCHARSET = 1 // return (wincodepage+1) or 0 for default (This encoding is used by stdout and the notify messages)
   };
 #endif
 }
diff --git a/Source/makenssi.cpp b/Source/makenssi.cpp
index a4a284f5..46abe55f 100644
--- a/Source/makenssi.cpp
+++ b/Source/makenssi.cpp
@@ -39,6 +39,7 @@ using namespace std;
 bool g_dopause=false;
 int g_display_errors=1;
 FILE *g_output;
+NStreamEncoding g_outputenc;
 #ifdef _WIN32
 UINT g_wincon_orgoutcp;
 #ifdef _UNICODE
@@ -290,7 +291,7 @@ static inline int makensismain(int argc, TCHAR **argv)
 
   HWND hostnotifyhandle=0;
   const TCHAR*stdoutredirname=0;
-  NStreamEncoding inputenc, outputenc;
+  NStreamEncoding inputenc, &outputenc = g_outputenc;
   int argpos=0;
   bool in_files=false;
   bool do_cd=true;
@@ -299,6 +300,8 @@ static inline int makensismain(int argc, TCHAR **argv)
   bool noconfig=false;
 #ifdef _WIN32
   signed char outputbom=1;
+
+  assert(CP_ACP == outputenc.GetCodepage()); // Required by CEXEBuild::notify() char* legacy handling.
 #endif
 
   // Some parameters have to be parsed early so we can initialize stdout and the "host API".
diff --git a/Source/utf.h b/Source/utf.h
index 3577782e..a57ca665 100644
--- a/Source/utf.h
+++ b/Source/utf.h
@@ -189,7 +189,7 @@ public:
   };
 
   NStreamEncoding() { Reset(); }
-  NStreamEncoding(WORD cp) { Reset();SetCodepage(cp); }
+  NStreamEncoding(WORD cp) { Reset(), SetCodepage(cp); }
   WORD GetCodepage() const { return m_cp; }
   void SetCodepage(WORD cp) { m_cp = cp; }
   void SafeSetCodepage(WORD cp)