KOI8-RU is compatible with KOI8-R in all Cyrillic Letters and completes it with four Ukrainian letters (#164, #180 - ukr. ie, #166, #182 - ukr. i, #167, #183 - ukr. yi, #173, #189 - ukr. ghe with upturn) which are compliant with ISO-IR-111.
All FORMS except positions ocupied by Ukrainian letters coincide with KOI8-R.
KOI8-U in Rusian, Ukrainian letters is completely compatible with ISO-IR-111 but differs in positions of one additional Ukrainian letter GHE WITH UPTURN and positions #128-#159 are used for FORMS elements from KOI8-R.