diff --git a/libcxx/include/__format/unicode.h b/libcxx/include/__format/unicode.h
--- a/libcxx/include/__format/unicode.h
+++ b/libcxx/include/__format/unicode.h
@@ -105,7 +105,7 @@
   requires same_as<iter_value_t<_Iterator>, char>
 _LIBCPP_HIDE_FROM_ABI constexpr bool __is_continuation(_Iterator __char, int __count) {
   do {
-    if ((*__char & 0b1000'0000) != 0b1000'0000)
+    if ((*__char & 0b1100'0000) != 0b1000'0000)
       return false;
     --__count;
     ++__char;
diff --git a/libcxx/test/std/utilities/format/format.functions/escaped_output.unicode.pass.cpp b/libcxx/test/std/utilities/format/format.functions/escaped_output.unicode.pass.cpp
--- a/libcxx/test/std/utilities/format/format.functions/escaped_output.unicode.pass.cpp
+++ b/libcxx/test/std/utilities/format/format.functions/escaped_output.unicode.pass.cpp
@@ -504,6 +504,13 @@
               "\xf7\xbf\xbf"
               "a");
 
+  // http://unicode.org/review/pr-121.html
+  test_format(R"("a\x{f1}\x{80}\x{80}\x{e1}\x{80}\x{c2}b")"sv,
+              "{:?}",
+              "a"
+              "\xf1\x80\x80\xe1\x80\xc2"
+              "b");
+
   // Code unit out of range
   test_format(R"("\u{10ffff}")"sv, "{:?}", "\xf4\x8f\xbf\xbf");               // last valid code point
   test_format(R"("\x{f4}\x{90}\x{80}\x{80}")"sv, "{:?}", "\xf4\x90\x80\x80"); // first invalid code point
diff --git a/libcxx/test/std/utilities/format/format.functions/unicode.pass.cpp b/libcxx/test/std/utilities/format/format.functions/unicode.pass.cpp
--- a/libcxx/test/std/utilities/format/format.functions/unicode.pass.cpp
+++ b/libcxx/test/std/utilities/format/format.functions/unicode.pass.cpp
@@ -272,6 +272,31 @@
     check(SV("*ZZZZ\xefZZZZ*"), SV("{:*^11}"), SV("ZZZZ\xefZZZZ"));
     check(SV("*ZZZZ\xffZZZZ*"), SV("{:*^11}"), SV("ZZZZ\xffZZZZ"));
 
+    // Invalid continuations
+    check(SV("\xc2\x00"), SV("{}"), SV("\xc2\x00")); // 0b0000'0000
+    check(SV("\xc2\x40"), SV("{}"), SV("\xc2\x40")); // 0b0100'0000
+    check(SV("\xc2\xc0"), SV("{}"), SV("\xc2\xc0")); // 0b1100'0000
+
+    check(SV("\xe0\x00\x80"), SV("{}"), SV("\xe0\x00\x80"));
+    check(SV("\xe0\x40\x80"), SV("{}"), SV("\xe0\x40\x80"));
+    check(SV("\xe0\xc0\x80"), SV("{}"), SV("\xe0\xc0\x80"));
+
+    check(SV("\xe0\x80\x00"), SV("{}"), SV("\xe0\x80\x00"));
+    check(SV("\xe0\x80\x40"), SV("{}"), SV("\xe0\x80\x40"));
+    check(SV("\xe0\x80\xc0"), SV("{}"), SV("\xe0\x80\xc0"));
+
+    check(SV("\xf0\x80\x80\x00"), SV("{}"), SV("\xf0\x80\x80\x00"));
+    check(SV("\xf0\x80\x80\x40"), SV("{}"), SV("\xf0\x80\x80\x40"));
+    check(SV("\xf0\x80\x80\xc0"), SV("{}"), SV("\xf0\x80\x80\xc0"));
+
+    check(SV("\xf0\x80\x00\x80"), SV("{}"), SV("\xf0\x80\x00\x80"));
+    check(SV("\xf0\x80\x40\x80"), SV("{}"), SV("\xf0\x80\x40\x80"));
+    check(SV("\xf0\x80\xc0\x80"), SV("{}"), SV("\xf0\x80\xc0\x80"));
+
+    check(SV("\xf0\x00\x80\x80"), SV("{}"), SV("\xf0\x00\x80\x80"));
+    check(SV("\xf0\x40\x80\x80"), SV("{}"), SV("\xf0\x40\x80\x80"));
+    check(SV("\xf0\xc0\x80\x80"), SV("{}"), SV("\xf0\xc0\x80\x80"));
+
     // Premature end.
     check(SV("*ZZZZ\xef\xf5*"), SV("{:*^8}"), SV("ZZZZ\xef\xf5"));
     check(SV("*ZZZZ\xef\xf5ZZZZ*"), SV("{:*^12}"), SV("ZZZZ\xef\xf5ZZZZ"));