This study investigates the effect of “adverse” contexts, especially that of the consonant /S/, on labial parameters for French /i,y/. Five parameters were analysed: the height, width and area of lip opening, the distance between the corners of the mouth, as well as lip protrusion. Ten speakers uttered a corpus made up of isolated vowels, syllables and logatoms. A special procedure has been designed to evaluate lip opening contours. Results showed that the carry-over effect of the consonant /S/ can impede the opposition between /i/ and /y/ in the protrusion dimension, depending upon speakers.