Two acoustic studies of voice onset time (VOT) in sibilant-stop (ST) consonant clusters, produced by first language (L1) speakers of Polish, are presented. In the first, a baseline study of L1 Polish comparing ST clusters with initial singleton stops, a small degree of VOT shortening after /s/ was found for /p/, but not /t/. The second study compared ST productions by L1 Polish speakers of second language (L2) English across two levels of proficiency, speaking in both L1 and L2. Rather than shortening post-/s/ VOT, as is common in L1 English, speakers from both proficiency groups exhibited longer VOT in their L2 than in their L1. These results are consistent with the claim that Polish learners of English mistakenly apply the long VOT of L2 English singleton fortis stops in the post-/s/ position. A phonological interpretation of the results within the Onset Prominence framework is provided.