diff --git a/src/ecmascript/see/input.c b/src/ecmascript/see/input.c
index 5d3b91f53..cba243551 100644
--- a/src/ecmascript/see/input.c
+++ b/src/ecmascript/see/input.c
@@ -111,9 +111,9 @@ append_unicode_to_SEE_string(struct SEE_interpreter *interp,
 		/* TODO: Should this reject code points in the
 		 * surrogate range? */
 		SEE_string_addch(str, u);
-	} else if (u <= 0x10FFFF) {
-		SEE_string_addch(str, 0xD800 + ((u - 0x10000) >> 10));
-		SEE_string_addch(str, 0xDC00 + (u & 0x3FF));
+	} else if (needs_utf16_surrogates(u)) {
+		SEE_string_addch(str, get_utf16_high_surrogate(u));
+		SEE_string_addch(str, get_utf16_low_surrogate(u));
 	} else {
 		/* str->interpreter exists but is not documented, so don't
 		 * use it; use a separate @interp parameter instead.
@@ -131,14 +131,12 @@ SEE_string_to_unicode(struct SEE_interpreter *interp, struct SEE_string *S)
 	if (S->length < 1) {
 		SEE_error_throw(interp, interp->Error,
 				"String is empty");
-	} else if (S->data[0] < 0xD800 || S->data[0] > 0xDFFF) {
+	} else if (!is_utf16_surrogate(S->data[0])) {
 		return S->data[0];
 	} else if (S->length >= 2
-		   && S->data[0] >= 0xD800 && S->data[0] <= 0xDBFF
-		   && S->data[1] >= 0xDC00 && S->data[1] <= 0xDFFF) {
-		return 0x10000
-			+ ((S->data[0] & 0x3FF) << 10)
-			+ (S->data[1] & 0x3FF);
+		   && is_utf16_high_surrogate(S->data[0])
+		   && is_utf16_low_surrogate(S->data[1])) {
+		return join_utf16_surrogates(S->data[0], S->data[1]);
 	} else {
 		SEE_error_throw(interp, interp->Error,
 				"Invalid UTF-16 sequence");
diff --git a/src/ecmascript/spidermonkey/form.c b/src/ecmascript/spidermonkey/form.c
index 643503a07..6da9b49c2 100644
--- a/src/ecmascript/spidermonkey/form.c
+++ b/src/ecmascript/spidermonkey/form.c
@@ -998,9 +998,9 @@ unicode_to_jsstring(JSContext *ctx, unicode_val_T u)
 		 * surrogate range? */
 		buf[0] = u;
 		return JS_NewUCStringCopyN(ctx, buf, 1);
-	} else if (u <= 0x10FFFF) {
-		buf[0] = 0xD800 + ((u - 0x10000) >> 10);
-		buf[1] = 0xDC00 + (u & 0x3FF);
+	} else if (needs_utf16_surrogates(u)) {
+		buf[0] = get_utf16_high_surrogate(u);
+		buf[1] = get_utf16_low_surrogate(u);
 		return JS_NewUCStringCopyN(ctx, buf, 2);
 	} else {
 		return NULL;
@@ -1024,12 +1024,12 @@ jsval_to_accesskey(JSContext *ctx, jsval *vp)
 	/* This implementation ignores extra characters in the string.  */
 	if (len < 1)
 		return 0;	/* which means no access key */
-	if (chr[0] < 0xD800 || chr[0] > 0xDFFF)
+	if (!is_utf16_surrogate(chr[0]))
 		return chr[0];
 	if (len >= 2
-	    && chr[0] >= 0xD800 && chr[0] <= 0xDBFF
-	    && chr[1] >= 0xDC00 && chr[1] <= 0xDFFF)
-		return 0x10000 + ((chr[0] & 0x3FF) << 10) + (chr[1] & 0x3FF);
+	    && is_utf16_high_surrogate(chr[0])
+	    && is_utf16_low_surrogate(chr[1]))
+		return join_utf16_surrogates(chr[0], chr[1]);
 	JS_ReportError(ctx, "Invalid UTF-16 sequence");
 	return UCS_NO_CHAR;	/* which the caller will reject */
 }
diff --git a/src/intl/charsets.h b/src/intl/charsets.h
index 095fe48e3..c0452aff7 100644
--- a/src/intl/charsets.h
+++ b/src/intl/charsets.h
@@ -88,4 +88,16 @@ unsigned char *u2cp_(unicode_val_T, int, int no_nbsp_hack);
 void init_charsets_lookup(void);
 void free_charsets_lookup(void);
 
+/* UTF-16 encodes each Unicode character U+0000...U+FFFF as a single
+ * 16-bit code unit, and each character U+10000...U+10FFFF as a pair
+ * of two code units: a high surrogate followed by a low surrogate.
+ * The range U+D800...U+DFFF is reserved for these surrogates.  */
+#define is_utf16_surrogate(u)           (((u) & 0xFFFFF800) == 0xD800)
+#define is_utf16_high_surrogate(u)      (((u) & 0xFFFFFC00) == 0xD800)
+#define is_utf16_low_surrogate(u)       (((u) & 0xFFFFFC00) == 0xDC00)
+#define join_utf16_surrogates(high,low) (0x10000 + (((high) - 0xD800L) << 10) + ((low) - 0xDC00))
+#define needs_utf16_surrogates(u)       ((uint32_t) ((u) - 0x10000) < 0x100000)
+#define get_utf16_high_surrogate(u)     (0xD800 + (((u) - 0x10000) >> 10))
+#define get_utf16_low_surrogate(u)      (0xDC00 + ((u) & 0x3FF))
+
 #endif