Two parallel acoustic analyses were performed for French and English sibilant sequences, based on comparably structured read-speech corpora. They comprised all sequences of voiced and voiceless alveolar and postalveolar sibilants that can occur across word boundaries in the two languages, as well as the invidivual alveolar and postalveolar sibilants, combined with preceding or following labial consonants across word boundaries. The individual sibilants provide references in order to determine type and degree of place assimilation in the sequences. Based on duration ratios and centre-of-gravity values that were determined for each individual sibilant and sibilant sequence, we found clear evidence for place assimilation not only for English, but also for French. In both languages the assimilation manifested itself gradually in the time as well as in the frequency domain. However, while in the English assimilation occurred strictly regressive and primarily towards postalveolar, the French assimilation was solely towards postalveolar, but in both directions regressive and progressive. Apart from these basic differences, the degree of assimilation varied in French and English due to the individual subjects, the vowel contexts, the frequencies of the target words, and the voicing features of the sibilants in ways that match well with previous findings.