diff --git a/compiler/ninl.pas b/compiler/ninl.pas
index 3f732b6b3a..2e1c91cf8f 100644
--- a/compiler/ninl.pas
+++ b/compiler/ninl.pas
@@ -507,6 +507,11 @@ implementation
                     else
                       begin
                         name := procprefixes[do_read]+'boolean';
+
+                        { iso pascal needs a different handler }
+                        if (m_iso in current_settings.modeswitches) and not(do_read) then
+                           name:=name+'_iso';
+
                         readfunctype:=booltype;
                       end
                   else
@@ -778,7 +783,6 @@ implementation
         para,nextpara:Tcallparanode;
         p1:Tnode;
         temp:Ttempcreatenode;
-
     begin
       found_error:=false;
       para:=Tcallparanode(params);
diff --git a/rtl/inc/compproc.inc b/rtl/inc/compproc.inc
index 1deaa00f79..b7e569c796 100644
--- a/rtl/inc/compproc.inc
+++ b/rtl/inc/compproc.inc
@@ -485,6 +485,7 @@ procedure fpc_write_text_enum(typinfo,ord2strindex:pointer;len:sizeint;var t:tex
 Procedure fpc_Write_Text_Currency(fixkomma,Len : Longint;var t : Text;c : Currency); compilerproc;
 {$endif FPC_HAS_STR_CURRENCY}
 Procedure fpc_Write_Text_Boolean(Len : Longint;var t : Text;b : Boolean); compilerproc;
+Procedure fpc_Write_Text_Boolean_Iso(Len : Longint;var t : Text;b : Boolean); compilerproc;
 Procedure fpc_Write_Text_Char(Len : Longint;var t : Text;c : Char); compilerproc;
 {$ifdef FPC_HAS_FEATURE_WIDESTRINGS}
 Procedure fpc_Write_Text_WideChar(Len : Longint;var t : Text;c : WideChar); compilerproc;
diff --git a/rtl/inc/text.inc b/rtl/inc/text.inc
index bcfb2841ed..ebbe757563 100644
--- a/rtl/inc/text.inc
+++ b/rtl/inc/text.inc
@@ -848,6 +848,18 @@ Begin
 End;
 
 
+Procedure fpc_Write_Text_Boolean_Iso(Len : Longint;var t : Text;b : Boolean); iocheck; compilerproc;
+Begin
+  If (InOutRes<>0) then
+   exit;
+{ Can't use array[boolean] because b can be >0 ! }
+  if b then
+    Write_Str(Len,t,'true')
+  else
+    Write_Str(Len,t,'false');
+End;
+
+
 Procedure fpc_Write_Text_Char(Len : Longint;var t : Text;c : Char); iocheck; compilerproc;
 Begin
   If (InOutRes<>0) then