Domain-initial strengthening has primarily been studied for consonants. This paper examines whether vowels also undergo boundary-induced phonetic variation and questions how this effect interacts with phonological contrast in a dense vowel system such as that of French. The labial articulation and the acoustic properties of the 10 French oral vowels /i, e, ɛ, a, y, ø, oe, u, o, ɔ/ are examined in Intonational Phrase-initial vs. Word-initial position. The vowels' phonetic properties are found to be affected by position but not in a uniform way. First, while all vowels are found to have a larger lip opening and width in IP-initial position, the effect is larger and more robust for unrounded vowels than rounded vowels leading to an enhanced distinction between vowels contrasting in rounding. No effect is found on lip protrusion. The distinction between these vowels is also found to be increased in IP-initial position by the enhancement of the spectral characteristics making unrounded vowels more 'unrounded-like' and – to a lesser degree – by the enhancement of the properties making rounded vowels more 'rounded-like'. The contrast between front and back vowels is also maximized by a tendency toward a higher F2 for front vowels and a lower F2 and F2–F1 for back vowels. Open and mid-open vowels also tend toward a higher F1. These results suggest that initial strengthening indeed contributes to maximizing phonetic contrasts between vowels in IP-initial position.