While assimilation was initially regarded as a categorical replacement of phonemes or phonological features, subsequent detailed phonetic analyses showed that assimilation actually generates a wide spectrum of intermediate forms in terms of speech timing and spectrum. However, the focus of these analyses predominantly remained on the assimilated speech sound. In the present study we go one step ahead in two ways. First, we look at acoustic phonetic detail that differs in the French vowels /i, a, u/ preceding single /s/ and /∫/ sibilants as well as /s#∫/ and /∫#s/ sibilant sequences. Second, our vowel measurements include not only F1 and F2 frequencies, but also traditional prosodic parameters like duration, intensity and voice quality. The vowels and sibilants were recorded as the central part of CVC#CVC pseudo-names in a contextualized read-speech paradigm. In the single-sibilant conditions we found that the vowels preceding /∫/ were longer, breathier, less intense, and had more cardinal F2 values than before /s/. For the /s#∫/ and /∫#s/ conditions we found regressive and progressive /s/-to-[∫] assimilation that was complete in terms of spectral centre-of-gravity measurements, although French is said to have only voice assimilation. Moreover, the vowels preceding the /s#∫/ sequences still bear an imprint of /s/ despite the assimilation towards [ ∫∫]. We discuss the implications of these findings for the time window and the completeness of assimilation as well as for the basic units in speech communication.