This document provides information about character encoding KOI8-C (KOI8 Cyrillic) proposed for use with Russian (including old orthography), Ukrainian, Belorussian, Serbian, Macedonian languages with special punctuation marks. KOI8-C is compatible with KOI8-R [1] and KOI8-U [2] in the area of Russian, Ukrainian and Belorussian letters, and extends these with letters for old Russian orthography, Yugoslavian cyrillic letters and typographical symbols in positions compatible with CP1251 for use in legacy applications.