The article analyzes a Mongolian translation of Ārya Bhadracarya Praṇidhāna Rāja, one of the popular Buddhist compositions of the devotional genre. Investigating Sanskrit, Uighur, Tibetan and Chinese loan words, the paper examines some cases of phonetic interference, i. e., ways of assimilation of such words in the Mongolian language. Most of the examined Sanskrit and Central Asian loan words had been borrowed into Mongolian via the Uighur language, Mongolian translators thus making use of the already developed Buddhist terms. The assimilation of loan words in written Mongolian is accompanied by the phonetic phenomena as follows: epenthesis, syncope, progressive and regressive assimilation, metathesis, uvularization of consonants, and transformation of some sounds. In most cases it is all due to the specific features of Mongolian phonetics, e. g., no consecutive use of three consonants is possible which results in the emergence of additional vowels between the consonants or interchange of letters in a word. The paper notes that two or even more of the mentioned phonetic phenomena may coexist in just one word. The considered translated text dated approximately to the 16th-17th cc. contains some features of pre-classical Mongolian.