Causal prepositions in the Russian language differ stylistically: the prepositions of this group are uneven in functions in different speech styles. Only some of them are stylistically neutral and function both in colloquial and in book speech, and everything else function in book speech styles. This is primarily due to active development of business writing in the XVIII century. This article deals with the possibility of functioning of the Russian causal prepositions in a particular style of speech andthe scope of their use; the stylistic factor of the inadmissibility of synonymous substitutions among these prepositions is found out.