The present paper deals with the problem of quality and quantity change (reduction) of unstressed vowels in Russian depending on their position relative to the stressed syllable and to the word boundaries. Quality change is measured by calculating the probability of vowel omission or replacement by another vowel phoneme. The study is based on a large corpus of read speech (over 30 hours). The results show that (1) there are two clear degrees of vowel reduction; (2) pre-stressed and post-stressed parts of the word are not symmetrical in terms of vowel reduction; (3) in pre-stressed syllables vowel quality change is correlated with quantity change.