EN
Since N. S. Trubeckoj's introduction of the notion of the Pre-Church Slavonic Language there have been continuous attempts to isolate and localise аll those elements of the Old Church Slavonic linguistic canon which were incorporated into it as a result of multiple copying by the 11th century writers done in various parts of the Slavonic world. Slavic scholars utilize different research methods which are characterized briefly in the reporting section of the paper. In the main part the author shows of what importance for the determination of Cyril and Methody's mother dialect are the results of the dialectological research obtained by V. Grigorovic in the mid-19th century and by M. Małecki in the period between the World Wars in the area of their home-town Solun (Salonika). This section is followed by a detailed presentation of old nasals, jat', sonants, the palatalization of fricative consonants and c, as well as the behavior of other consonants before front vowels.