TheCompatibility Encoding Scheme for UTF-16: 8-Bit (CESU-8) is a variant ofUTF-8 that is described inUnicode Technical Report #26.^[1] A Unicode code point from theBasic Multilingual Plane (BMP), i.e. a code point in the rangeU+0000 toU+FFFF, is encoded in the same way as in UTF-8. A Unicode supplementary character, i.e. a code point in the rangeU+10000 toU+10FFFF, is first represented as a surrogate pair, like inUTF-16, and then each surrogate code point is encoded in UTF-8. Therefore, CESU-8 needs six bytes (3 bytes per surrogate) for each Unicode supplementary character while UTF-8 needs only four. Though not specified in the technical report,unpaired surrogates are also encoded as 3 bytes each, and CESU-8 is exactly the same as applying an olderUCS-2 to UTF-8 converter to UTF-16 data.

The encoding of Unicode non-BMP characters works out to11101101 1010yyyy 10xxxxxx 11101101 1011xxxx 10xxxxxx (yyyy represents the top five bits of the character minus one). The byte values0xF0—0xF4 will not appear in CESU-8, as they start the 4-byte encodings used by UTF-8.

CESU-8 is not an official part of the Unicode Standard, because Unicode Technical Reports are informative documents only.^[2] It should be used exclusively for internal processing and never for external data exchange. Supporting CESU-8 inHTML is prohibited by theW3C^[3] andWHATWG.^[4]

Java'sModified UTF-8 is CESU-8 with a special overlong encoding ofNULU+0000 as the two-byte sequenceC0 80.^[5] TheOracle database uses CESU-8 for itsUTF8 character set while standard UTF-8 is calledAL32UTF8 (since Oracle version 9.0).^[6]

• Unicode Technical Report #26
• Modified UTF-8 definition
• Graphical View of CESU-8 in ICU's Converter Explorer

• Unicode Transformation Formats
• Character encoding

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