In this study we investigate vowel reduction and the role of some lexical factors in the production of vowels extracted from a corpus of French conversations. Vowel durations and spectral quality are examined with respect to 1. their interaction in the corpus, 2. the position of vowels in words, and 3. word frequency and word category. The analyses are conducted on vowels produced by 16 speakers. Our study provides strong evidence that vowel reduction (decrease in durations and more centralized spectral values) affects most of the vowels in conversational speech. The results show that vowels in final syllables of words were less often reduced while the preceding ones show reduced durations and centralized formant values. Moreover vowels are more reduced in monosyllabic function words than in monosyllabic content words. Nevertheless, we did not find a clear effect of word frequency on vowel durations. Finally, our study shows that vowel reduction depends on several factors related to lexical properties (word category) and to prosodic properties (stress and final lengthening).