Introduction. The article examines some phonetic and morphological features of the Kalmyk language traced in written monuments published by the famous Mongolists John Krueger and Robert Service in the ‘Kalmyk Old-Manuscript Documents of Isaac Jacob Schmidt’ (2002). The book presents facsimiles, transcriptions and translations of epistolary documents into English, which cover the period from 1800 to 1810. Most of the letters — 54 out of 80 — reflect correspondence with the Baga Dorbet Princes Tundutovs, including 18 from Erdeni Taisha, 27 from Tsebek, and 9 more from Jamba Taisha. These written monuments are kept in the archives of the Moravian Brothers Community in Herrnhut (Unitas Fratrum, Eng. ‘United Brothers’) in folder R.15.R.IIa No. 5. Goals. The paper aims at investigating some phonetic and morphological features of the then Kalmyk language discovered in the published letters. Results. The language of I. H. Schmidt’s letters reflects the stage in the development of the Kalmyk language in the late 18th – early 19th centuries characterized by a natural convergence of the phonetic and grammatical norms inherent to the written literary language and living folk speech. The materials of the texts show that the beginning of the 19th century witnessed a process of transition of the combinations -ou and -öü into the long vowels [u:] and [ü:], respectively. It is also noted that in words where the diphthong -iu is historically (traditionally) used, e.g., in the word alčiur — in letters it is transmitted as alčuur, which meets the requirements of live pronunciation. In the language of letters of I. J. Schmidt, when it comes to construct the imperative form of the 2nd person plural, several cases of using the colloquial formant -tan are recorded, instead of the traditional indicator -qtün / -qtun. The past participle, which is historically formed with -qsan / -qsen affixes, also approaches colloquial forms, therefore, there are frequent cases of using -san / -sen forms, which are closer to modern Kalmyk (e.g., xara torγon irsen bolxuna ‘if the black silk arrives’). Linguistic processes associated with the reduction of short vowels of non-first syllables of a word, reduction of morphological formants, etc., are reflected to a greater or lesser extent in the text of the studied monument. Conclusions. Thus, the language of I. Ya. Schmidt’s letters reflects the stage of development of the Kalmyk language at the end of the 18th – beginning of the 19th centuries characterized by a natural convergence of phonetic and grammatical norms of the written literary language and living folk speech.