A hypothesis is proposed which claims that vowel prosthesis before word-initial [sC] clusters by L1 Spanish learners of English is a perception-based repair phenomenon conditioned by L1 biases for native patterns of intergestural timing in complex onsets. Data from a series of off-line perception tests involving fifty L1 Spanish-speaking children confirm the erroneous perception of an extra syllable in a majority of nonce tokens containing the target stimuli. Results of a set of follow-up discrimination and identification tasks prove that Spanish-speakers access information derived from native timing patterns in order to map continuous speech streams to categorical units before lexical storage occurs, supporting the viability of a bias derived from native timing configurations. The findings are of interest since they bear directly on contemporary research in the acquisition of L1 phasing relations and their effects on L2 phonological acquisition.