When native Spanish speakers produce English words with initial [s]-consonant clusters (sC), they sometimes produce a prothetic vowel, e.g. stigma > estigma. This paper reports a production experiment on this phenomena, as well as computational modelling of the experimental results. Carlisle (1991a) proposed the ‘resyllabification account’ in which prothesis is a language transfer effect, whose essential motivation is to satisfy L1/Spanish syllable phonotactics. Replicating all previous work, a greater rate of prothesis was found in postconsonantal contexts than in postvocalic contexts (Rick (e)stinks > Ricky (e)stinks). A novel prediction is that when prothesis occurs, the [s] should have durational characteristics associated with the coda position, whereas it should have onset characteristics when prothesis does not occur; this was found. Another prediction is that a grammar which captures the variability in prothesis should in some sense be “between” the L1/Spanish and L2/English grammars. This latter prediction was tested by developing a constraint-based analysis of sC prothesis in Maximum Entropy Harmonic Grammar (Goldwater & Johnson, 2003). The results were consistent with a view of language transfer as ‘linear interpolation’ of constraint weights, conditioned on an ‘effort’ constraint reflecting how phonological planning varies with task/ modality demands.