Based on the examination of 385K vowels pertaining to 6 oral and 3 nasal vowel categories produced by twenty-three French speakers in both read and spontaneous speech, the present study questions the interplay between intra-speaker style-dependent variability in vowel production and speaker-specific vowel properties. Acoustic properties of the speakers’ vowels in the 12-D MFCC space are compared to that of other speakers in two styles. Results show that vowels do index speaker-distinctiveness better in read vs. spontaneous speech. Furthermore, in both speech styles, distinctions between speakers are the largest for the nasal vowels. Intra-speaker variability in vowel production is also examined between speech styles and is found to depend on the speaker and on the vowel category. However, for most speakers and most vowels, the variation between styles is smaller than the distinction between speakers in both styles. Implications of these results for speaker identification are discussed.