This paper investigates how prosodic boundary strength (IPi vs. IPm) affects the production of the vowels /i, e, ɛ, a/, which contrast on a four degrees of height scale in French. Acoustic and tongue configuration data are examined for four speakers. Results show an expansion of the vowel space in IP-initial position that is achieved by a rising of F2 for /i, e, ɛ/ and of F1 for /ɛ, a/. Differences in tongue configuration also contribute to an expansion of the articulatory space in IP-initial position are also observed. Measurements on the highest point of the tongue show a narrowing of the constriction for /i, e, ɛ/, accompanied by fronting for /e, ɛ/, and a widening and backing of the constriction for /a/ for most speakers. These variations in IP-initial position lead to a maximization of phonetic contrasts in terms of height and frontness within the pairs /e-ɛ/ and /ɛ-a/ for most speakers, but not within the pair /i, e/, probably due to articulatory/acoustic constraints.