The unrounded Russian vowel phonemes are occasionally pronounced as rounded in unstressed positions. This can be observed when one of the following vowels is rounded. The aim of current research is to define how strong this phonological process is and what factors trigger the process. The experimental results showed this tendency in speech of young adults. Vowel labialization occurs in about a third of non-immediately-pretonic and post-tonic syllables in connected speech. While the roundedness of vowels does not affect word intelligibility, it is clearly perceived when vowels are perceived in isolation or within a syllable. The tendency should be taken into account in developing probabilistic dictionaries for automatic speech recognition and G2P procedures.