Abstract
This article presents a study about the usage of short and long forms of adjectives in the literary Old Russian. It is a diachronic study that compares three historical periods: 1) the end of XIIth century, 2) the end of XIVth century -- the first half of XVth century and 3) the first half of and the mid-XVIIth century. The coexistence of two forms is treated in the light of grammaticalization theory, according to which more ancient linguistic structures (short adjectives) persist together with innovative ones, fruits of grammaticalization, (long adjectives), when there are differences in their usage. The variation of the usage of two adjective forms is explained on the basis of statistical analysis of the data which has indicated the linguistic factors relevant to the choice between them.