It has long been known that a number of works of the Byzantine literature survives only in the Church Slavonic translations. However, a systematic approach to the collections within which they circulated may yield very interesting and often unexpected results. The article analyses the texts from the ten-volume menological collection widely known in Old Rus' according to four categories. The first includes the works lost in the original without a trace. The second is represented by the texts lost in Greek which nevertheless left traces in contemporary or later sources. The third group consists of the texts which survive in the original with greater or lesser losses, while the fourth - of the works available, besides the translation, in one or more Greek manuscripts. Every time it turns out that the Slavonic translations are extremely valuable not only from the point of view of textology, but also of their contents. Therefore full-fledged study of the Byzantine history and culture especially of the 8th to 10th century is impossible without use of the sources that survive only in Church Slavonic. Moreover, reconstruction of such substantial collections of texts as, e.g., the full cycle of pre-Metaphrastic menologia is only possible if the Old Russian literature is taken into account. The underlying reason is that the source for the translations was one of the richest book deposits of the Empire - the library of the Studios monastery in Constantinople.