Acoustic and perceptual studies investgate B2-level Polish learners’ acquisition of second language (L2) English word-boundaries involving word-initial vowels. In production, participants were less likely to produce glottalization of phrase-medial initial vowels in L2 English than in first language (L1) Polish. Perception studies employing word monitoring and word counting tasks found that glottalization of word-initial vowels had a negligible impact on the processing of L2 word boundaries. Taken together, these experiments suggest that B2-level learners are relatively successful in acquiring word-boundary linking processes that are for the most part absent from L1 Polish, and challenge the notion of an L2 ‘Word Integrity’ constraint. The cross-language interactions observed in these experiments are compatible with the claim that the realization of word-initial vowels is governed by a representational parameter, which may be derived in the Onset Prominence framework.