The issue of morphonology status and units of its description remains relevant in modern linguistics. The purpose of study is to describe the morphonological rules for choosing suffix -ost’ (-est’), to investigate the hierarchies of morphonological units that determine the synthesis of the sound structure of derivatives under study, consisting of a set of stress morphonemes, pho-nemes alternation and allomorphs alternation. We mainly use the descriptive method, which in-cludes such techniques as observation, comparison, juxtaposition, generalization, interpretation. Based on the description, we conclude that word formation of considered class of words is distin-guished by fusion, and not agglutination. The choice of an allomorph depends on morphological characteristics of generator, and allomorph determines the stress and the type of derivative's alter-nant, that is, the morphological unit includes phonemes alternation, stress alternation and allo-morphs alternation of the suffix, which are 3 morphonological units. The results of study, in par-ticular, the significance of suffix in describing the theoretical foundations for describing the mor-phonological phenomena of Russian language, as well as the model of morphonological descrip-tion of Russian word formation, make a certain contribution to the development of morphonology as an independent science. The practical significance lies in the fact that the conclusions and materials of study can be used in the compilation of special morphonological dictionaries.