The present paper is concerned with the adverbial and semi-adverbial phrases (with incomplete lexicalization), which are formed by prefixing a preposition to an autosemantic word in the Old Slavonic language. The empirical material is systematized on the basis of the prepositions involved in the word formation process, with regard to the adjuncts determined by the newly-formed adverbs and on the basis of the lexico-grammatical affiliation of the motivating word. The path of adverbialization is analyzed in each case and the systematic relations of the newly-formed adverbial units are also described.