This study presents acoustic and electro-magnetic articulography (EMA) data for the back rounded vowel /u/ in preboundary position in French. Five boundary types are examined: the Utterance, the Intonational phrase, the Accentual phrase, the Word and the Syllable. The three speakers studied produce similar acoustic output, with both F1 and F2 becoming lower before stronger prosodic boundaries. However, the Utterance boundary has a particularly strong effect on F1, which is particularly low before this boundary. To achieve the acoustic output observed, the speakers adopt different articulatory strategies at different prosodic boundaries. The strategies observed before the strongest boundaries are tongue dorsum backing (coupled with either raising or lowering, depending on the speaker); tongue tip retraction; and lip protrusion. Somewhat unexpectedly in light of acoustic considerations, lip constriction is observed to be greater before the weaker prosodic boundaries. This result, considered in conjunction with the tongue data and with the lip protrusion data, leads us to suggest that the French speakers in our study are actively aiming to prevent F2 from becoming too high before the weaker prosodic boundaries. We suggest that a high F2 for /u/ may lead to perceptual confusion with the front rounded vowel /y/, which is also present in the French phoneme inventory. This result echoes our previous results for the front unrounded vowel /i/ (Tabain & Perrier, 2005. Articulation and acoustics of /i/ in preboundary position in French. Journal of Phonetics, 33, 77–100), and suggests that the structure of a language's phoneme inventory has important effects on the articulatory strategies adopted by its speakers.