The paper discusses the status of the formant z-, represented in the vocabulary of Arkhangelsk dialects of Russian. It mostly occurs on verbs, less often on words of other parts of speech. Consideration of its status concerns several aspects, such as phonetic position, semantics, correlation with synonymous prefixes, and area of usage. The formant z- in Arkhangelsk dialects is used both before obstruents and sonorants and /v-vʲ/. In the process of functioning, it moves from phonetics to morphemics, expanding the range of its meanings and, as a result, acquiring the capability to be used analogically to the prefixes vz-, za-, iz-, s-. The correlation between the semantics of z- and vz- is explained by the phonetic simplifi cation of [vz] before a consonant, whereby the resulting z- inherits all the meanings of vz-. In turn, the prefixes vz-, iz-, za-, s- show semantic commonalities, and this explains the possibility of formations with z- synonymous not only to formations with vz-, but also with iz-, za-, s- in the following meanings: ‘committing an action, bringing an action to the desired limit, result’ — a meaning common with the prefixes vz-, iz-, za-, s-; ‘upward direction of an action or movement’ — a meaning common with the prefixes vz-, za-; ‘starting an action’ — a meaning common with the prefixes vz-, za-; ‘performing a single action’ — a meaning common with the prefix vz-. Thus, the formant z- receives the status of a prefix, synonymous in all meanings to the prefix vz- and partly synonymous to the prefixes iz-, za-, s-. The conclusions about the genesis of the prefix z-, based on the analysis of the material of the Arkhangelsk dialects, are confirmed by the data of dictionaries of other Russian dialects, including those that are not in direct contact with the Belarusian and Ukrainian languages. The results of the study correlate with the problem of the general arealogy of Slavic dialects, aimed at revealing the relations between different languages, which represent their comparability.