Purpose: Phonetic variation due to domain initial strengthening was investigated with respect to the acoustic and articulatory distinctiveness of vowels within a subset of the French oral vowel system /i, e, ɛ, a, o, u/, organized along 4 degrees of height for the front vowels and 2 degrees of backness at the close and midclose height levels. Method: Acoustic and lingual ultrasound data were examined to characterize the production of vowels in intonational phrase initial position, compared with intonational phrase medial position, for 4 speakers. Results: Formant values and estimates of lingual constriction location and degree differed according to the prosodic position independent of vowel duration, with a higher F1 for /a/, a higher F2 for /ɛ/, a backer constriction for /o/ and /a/ but a fronter constriction for /ɛ/, and a narrower constriction for /e, ɛ, u, o/ but a wider constriction for /a/. For most speakers, these variations enlarge the acoustic and/or articulatory distance between members of the pairs /e-ɛ/, /ɛ-a/, /u-o/, /i-u/, and /e-o/ but reduce the distinction within the pair /i-e/. Conclusions: These changes in intonational phrase initial position are vowel dependent and frequently contribute to augmenting the phonetic distinctiveness between vowels contrasting along the height and backness dimensions