The aim of the study is to identify the potential phonetic changes of Russianisms included in the substandard of the Tajik language. The article examines phonetic changes (replacements and transitions of vowels and consonants, permutations, insertions of missing sounds, prosthetic and epenthetic phenomena, etc.) of Russian words included in the substandard of the Tajik language at different stages of its development, and determines phonetic transformations of Russianisms characteristic of certain historical periods. The scientific novelty consists in the fact that for the first time a comparative study of Russian borrowings in diachronic aspect was conducted, i.e., phonetic and graphic adaptation of Russianisms in different periods of their entry into the Tajik language was considered. Periodization of the process of lexical borrowing gives an opportunity to consider the nature of sound changes in the phonetic system of the Tajik language in different historical periods. The comparative analysis of Russian borrowings in diachrony covers three historical periods: 1) the late 19th and early 20th cc. – the pre-revolutionary period; 2) 1920s-1990s – the Soviet period; 3) 1990s and up to the present time – the post-Soviet period. As a result of the study, it was found that over a short period of time, Russian lexical borrowings have undergone qualitative and quantitative changes in the phonetic form in Tajik, which is a characteristic feature of such phenomena in the host language. Comparative analysis has shown that Russianisms on the territory of Tajikistan have undergone various adaptations depending on the region, since a single phonetic norm of the Tajik language has not been formed.