This study is a cross-linguistic investigation of qualitative and quantitative variations due to 1/ the structure of vocalic system, 2/ the amount of context within speech message. We hypothesize that phonetic distinctivity of vowels in a language is relative to 1/ the properties of the phonological system, 2/ the amount of informational context. Three languages (Spanish, French and English) were analyzed in three different types of speech (isolated vowels, within words and within texts). Results show 1/ centralization in the three vocalic systems relative to the amount of context, 2/ an increase of vowel dispersion also due to an increase of context information.