Voicing assimilations inside groups of obstruents occur in opposite directions in French and German, where they are respectively regressive and progressive. The aim of the study is to investigate (1) whether non native speakers (here French learners of German) are apt to acquire subtle L2 specificities like assimilation direction, although they are not aware of their very existence, or (2) whether their productions depend essentially upon other factors, in particular consonant place of articulation. To that purpose, a corpus made up of groups of obstruents (/t/ followed by /z/, /v/ or /f/) embedded into sentences has been recorded by 16 French learners of German (beginners and advanced speakers). The consonants are separated by a word or a syllable boundary. Results, derived from the analysis of consonant periodicity and duration, do not stand for an acquisition of progressive assimilation, even by advanced speakers, and do not show differences between the productions of advanced speakers and beginners. On the contrary the boundary type and the consonant place of articulation play an important role in the presence or absence of voicing inside obstruent groups. The role of phonetic, universal mechanisms against linguistic specific rules is discussed to interpret the data.