The representation of L2 words and non-words was analysed in a series of three experiments. Catalan-Spanish bilinguals, differing in terms of their L1 and the age of exposure to their L2 (since birth-simultaneous bilinguals-or starting in early childhood-early sequential bilinguals), were asked to perform a lexical decision task on Catalan words and non-words. The non-words were based on real words, but with one vowel changed: critically, this vowel change could involve a Catalan contrast that Spanish natives find difficult to perceive. The results confirmed previous data indicating that in spite of early, intensive exposure, Spanish-Catalan bilinguals fail to perceive certain Catalan contrasts, and that this failure has consequences at the lexical level. Further, the results from simultaneous bilinguals show: (a) that even in the case of bilinguals who are exposed to both languages from birth, a dominant language prevails; and (b) that simultaneous bilinguals do not attain the same level of proficiency as early bilinguals in their first language.