This article is devoted to an analysis of some specific language features found in a num¬ber of Yenisei runic epitaphs and differing from the standards of the supra-dialectic Old Turkic koine, the language of the written monuments of ancient Turkic ethnic groups, which spoke different Turkic varieties and dialects by the 8th century. Detection and analysis of non-standard words and forms in the Yenisei epitaph texts indicate the idioms, that the creators of these texts apparently spoke. Further, we can identify the ethnicity of the person to whom an inscription was dedicated and link the inscription to a certain eth¬nic group and its ancient state, about which the inscriptions either are silent or make am¬biguous statements. Non-standard language features, presented in this article for one group of the Yenisei inscriptions, indicate their possible connection with the language of Kyrghyz; features of another group of these inscriptions show relation to a language of the Tuvan type.