This study focuses on voicing assimilation across word boundaries in the speech of second language (L2) users. We compare native speakers of British English to speakers of two West Slavic languages, Czech and Slovak, which, despite their many similarities, differ with respect to voicing assimilation rules. Word-final voicing was analysed in 30 speakers, using the static value of voicing percentage and the voicing profile method. The results of linear mixed-effects modelling suggest an effect of first language (L1) transfer in all L2 English speaker groups, with the tendency to assimilate being correlated with the strength of foreign accent. Importantly, the two language groups differed in assimilation strategies before sonorant consonants, as a clear effect of L1-based phonetic influence.