diff --git a/Docs/src/attributes.but b/Docs/src/attributes.but
index 2205ecc2..66ac1f97 100644
--- a/Docs/src/attributes.but
+++ b/Docs/src/attributes.but
@@ -60,7 +60,7 @@ When used outside a \R{pageex}{PageEx} block: Sets the text for the titlebar of
 
 When used inside a \R{pageex}{PageEx} block: Sets the subcaption of the current page.
 
-Accepts variables. If variables are used, they must be initialized on \R{oninit}{.onInit}.
+Accepts variables. If variables are used, they must be initialized on \R{oninit}{.onInit} or \R{onguiinit}{.onGUIInit}.
 
 \S2{achangeui} ChangeUI
 
diff --git a/Docs/src/history.but b/Docs/src/history.but
index 42397a7b..00a49cff 100644
--- a/Docs/src/history.but
+++ b/Docs/src/history.but
@@ -18,6 +18,8 @@ Released on ? ?th, 2017
 
 \S2{} Minor Changes
 
+\b Variables used in Caption can now be set in .onGuiInit (\W{http://sf.net/p/nsis/feature-requests/539}{RFE #539})
+
 \b MultiUser now supports $ProgramFiles64 (\W{http://sf.net/p/nsis/bugs/843}{bug #843})
 
 \b Added support for MUI_UNCONFIRMPAGE_VARIABLE (\W{http://sf.net/p/nsis/feature-requests/511}{RFE #511}) and MUI_PAGE_CUSTOMFUNCTION_DESTROYED
diff --git a/Source/exehead/Main.c b/Source/exehead/Main.c
index d5eb0686..dc2e0fe9 100644
--- a/Source/exehead/Main.c
+++ b/Source/exehead/Main.c
@@ -58,7 +58,7 @@
 extern HANDLE dbd_hFile;
 #endif
 
-TCHAR g_caption[NSIS_MAX_STRLEN*2];
+TCHAR g_caption[NSIS_MAX_STRLEN*2]; // Why does this have to be NSIS_MAX_STRLEN*2?
 #ifdef NSIS_CONFIG_VISIBLE_SUPPORT
 HWND g_hwnd;
 HANDLE g_hInstance;
diff --git a/Source/exehead/Ui.c b/Source/exehead/Ui.c
index 6f7bf222..cb36e19e 100644
--- a/Source/exehead/Ui.c
+++ b/Source/exehead/Ui.c
@@ -170,6 +170,16 @@ void NSISCALL build_g_logfile()
 
 int *cur_langtable;
 
+static TCHAR* update_caption()
+{
+  TCHAR *gcap = g_caption;
+  GetNSISString(gcap, LANG_CAPTION);
+#ifdef NSIS_SUPPORT_BGBG
+  my_SetWindowText(m_bgwnd, gcap);
+#endif
+  return gcap;
+}
+
 static void NSISCALL set_language()
 {
   LANGID lang_mask=(LANGID)~0;
@@ -200,14 +210,9 @@ lang_again:
   }
 
   cur_langtable = selected_langtable;
-
   myitoa(state_language, *(LANGID*)language_table);
-  {
-    TCHAR *caption = GetNSISString(g_caption,LANG_CAPTION);
-#ifdef NSIS_SUPPORT_BGBG
-    my_SetWindowText(m_bgwnd, caption);
-#endif
-  }
+
+  update_caption();
 
   // reload section names
   {
@@ -581,7 +586,7 @@ nextPage:
         SetActiveCtl(m_hwndOK);
       }
 
-      mystrcpy(g_tmp,g_caption);
+      mystrcpy(g_tmp,update_caption());
       GetNSISString(g_tmp+mystrlen(g_tmp),this_page->caption);
       my_SetWindowText(hwndDlg,g_tmp);
 
diff --git a/Source/exehead/api.h b/Source/exehead/api.h
index 61591d4e..956f763a 100644
--- a/Source/exehead/api.h
+++ b/Source/exehead/api.h
@@ -36,8 +36,8 @@ enum NSPIM
 // Should always be __cdecl for future expansion possibilities
 typedef UINT_PTR (*NSISPLUGINCALLBACK)(enum NSPIM);
 
-// extra_parameters data structures containing other interesting stuff
-// but the stack, variables and HWND passed on to plug-ins.
+// extra_parameters data structure containing other interesting stuff
+// besides the stack, variables and HWND passed on to plug-ins.
 typedef struct
 {
   int autoclose;
@@ -83,4 +83,4 @@ typedef struct {
 // sent as wParam with WM_NOTIFY_OUTER_NEXT when user cancels - heed its warning
 #define NOTIFY_BYE_BYE 'x'
 
-#endif /* _PLUGIN_H_ */
+#endif /* _NSIS_EXEHEAD_API_H_ */
diff --git a/Source/exehead/util.c b/Source/exehead/util.c
index 39e55780..b28c830e 100644
--- a/Source/exehead/util.c
+++ b/Source/exehead/util.c
@@ -144,7 +144,7 @@ int NSISCALL my_MessageBox(const TCHAR *text, UINT type) {
   mbp.hwndOwner = g_hwnd;
   mbp.hInstance = g_hInstance;
   mbp.lpszText = text;
-  mbp.lpszCaption = g_caption;
+  mbp.lpszCaption = g_caption; // Should call update_caption() here?
   mbp.dwStyle = _type;
   
   return MessageBoxIndirect(&mbp);