Abstract The present study analyzes the transformation of the vowel system and especially the process of vowel mergers based on the Latin inscriptions of the Danubian provinces. With the help of the Computerized Historical Linguistic Database of the Latin Inscriptions of the Imperial Age ( http://lldb.elte.hu/ ), it tries to draw and then compare the phonological profiles of the selected provinces and to describe the dialectal position of the Danubian provinces regarding vocalism in the first four centuries AD. The analysis, which also covers comparisons with certain provinces of Italy and Dalmatia, is carried out considering four aspects: the ratio of vocalic versus consonantal changes, the ratio of vowel mergers compared to vocalic changes, the ratio of e-i and o-u mergers compared to each other, and the ratio of vowel mergers by stressed and unstressed syllable. As a result of the present study, it was revealed that Danubian provinces cannot be treated as a unit or as clearly separate from the other areas studied according to either aspect of the study. The Dacian development, which can only be observed in the 2 nd –3 rd century, can easily be placed among the Danubian provinces, so it is not necessary to connect it with the developmental trends in southern Italy. The present study, which continued József Herman's research, managed to explore the hitherto little-known linguistic and dialectological features of Latin in the Danubian provinces.