diff --git a/include/ruby/io.h b/include/ruby/io.h
index 048090c..737c509 100644
--- a/include/ruby/io.h
+++ b/include/ruby/io.h
@@ -98,7 +98,6 @@ typedef struct rb_io_t {
 } rb_io_t;
 
 #define HAVE_RB_IO_T 1
-
 #define FMODE_READABLE              0x00000001
 #define FMODE_WRITABLE              0x00000002
 #define FMODE_READWRITE             (FMODE_READABLE|FMODE_WRITABLE)
@@ -109,6 +108,7 @@ typedef struct rb_io_t {
 #define FMODE_APPEND                0x00000040
 #define FMODE_CREATE                0x00000080
 /* #define FMODE_NOREVLOOKUP        0x00000100 */
+#define FMODE_EXCL                  0x00000400
 #define FMODE_TRUNC                 0x00000800
 #define FMODE_TEXTMODE              0x00001000
 /* #define FMODE_PREP               0x00010000 */
diff --git a/io.c b/io.c
index fcccc71..8ae6bf4 100644
--- a/io.c
+++ b/io.c
@@ -4879,9 +4879,13 @@ rb_io_fmode_modestr(int fmode)
       case FMODE_READABLE:
 	return MODE_BTMODE("r", "rb", "rt");
       case FMODE_WRITABLE:
+	if (fmode & FMODE_EXCL)
+	    return MODE_BTMODE("wx", "wbx", "wtx");
 	return MODE_BTMODE("w", "wb", "wt");
       case FMODE_READWRITE:
 	if (fmode & FMODE_CREATE) {
+	    if (fmode & FMODE_EXCL)
+		return MODE_BTMODE("w+x", "wb+x", "wt+x");
 	    return MODE_BTMODE("w+", "wb+", "wt+");
 	}
 	return MODE_BTMODE("r+", "rb+", "rt+");
@@ -4928,6 +4932,11 @@ rb_io_modestr_fmode(const char *modestr)
 	  case '+':
             fmode |= FMODE_READWRITE;
             break;
+	  case 'x':
+	    if (!(fmode & FMODE_TRUNC))
+		goto error;
+	    fmode |= FMODE_EXCL;
+	    break;
 	  default:
             goto error;
 	  case ':':
@@ -4971,6 +4980,9 @@ rb_io_oflags_fmode(int oflags)
     if (oflags & O_CREAT) {
 	fmode |= FMODE_CREATE;
     }
+    if (oflags & O_EXCL) {
+	fmode |= FMODE_EXCL;
+    }
 #ifdef O_BINARY
     if (oflags & O_BINARY) {
 	fmode |= FMODE_BINMODE;
@@ -5006,6 +5018,9 @@ rb_io_fmode_oflags(int fmode)
     if (fmode & FMODE_CREATE) {
         oflags |= O_CREAT;
     }
+    if (fmode & FMODE_EXCL) {
+        oflags |= O_EXCL;
+    }
 #ifdef O_BINARY
     if (fmode & FMODE_BINMODE) {
         oflags |= O_BINARY;
@@ -5029,6 +5044,8 @@ rb_io_oflags_modestr(int oflags)
 #else
 # define MODE_BINARY(a,b) (a)
 #endif
+#define MODE_BINARY_EXCL(a,b,c,d) \
+ ((oflags & O_EXCL) ? MODE_BINARY(d, c) : MODE_BINARY(b, a))
     int accmode = oflags & (O_RDONLY|O_WRONLY|O_RDWR);
     if (oflags & O_APPEND) {
 	if (accmode == O_WRONLY) {
@@ -5044,8 +5061,10 @@ rb_io_oflags_modestr(int oflags)
       case O_RDONLY:
 	return MODE_BINARY("r", "rb");
       case O_WRONLY:
-	return MODE_BINARY("w", "wb");
+	return MODE_BINARY_EXCL("w", "wb", "wx", "wbx");
       case O_RDWR:
+	if (oflags & O_CREAT)
+	    return MODE_BINARY_EXCL("w+", "wb+", "w+x", "wb+x");
 	return MODE_BINARY("r+", "rb+");
     }
 }
diff --git a/test/ruby/test_file.rb b/test/ruby/test_file.rb
index a700527..66a521f 100644
--- a/test/ruby/test_file.rb
+++ b/test/ruby/test_file.rb
@@ -437,4 +437,16 @@ class TestFile < Test::Unit::TestCase
       assert_file.not_exist?(path)
     end
   end
+
+  def test_exclusive_mode
+    Dir.mktmpdir('feature11258') do |tmpdir|
+      fname = "#{tmpdir}/tmp.txt"
+      # file must be created and not just opened
+      assert_nothing_raised { File.new(fname, 'wx').close }
+      assert_raise(Errno::EEXIST) { File.new(fname, 'wx') }
+      # mode must not be read or append mode
+      assert_raise(ArgumentError) { File.new(fname, 'rx') }
+      assert_raise(ArgumentError) { File.new(fname, 'ax') }
+    end
+  end
 end