Paola Escudero & Paul Boersma The subset problem in L2 perceptual development: Multiple-category assimilation by Dutch learners of Spanish We define multiple-category assimilation as the pattern in foreign-language perception in which a binary contrast in the L2 is perceived as more than two categories in the first language. We show that the perceptual differentiation by learners of the target language is too good, in the sense that the learners perceive a category (or a contrast) that does not consist in the target language. As our test case, we consider the perception of Spanish vowels by Dutch learners of Spanish. Dutch has three short front vowels, whereas Spanish has only two short front vowels. We show that the extra Dutch category hampers the perceptual proficiency of the learners. It turns out (1) that Dutch learners of Spanish perceive the two Spanish front vowels as three different short Dutch vowels; (2) that these learners perceive the extra category less often if they think that the language they hear is Spanish than if they think the language they hear is Dutch; and (3) that the extent to which they lose this extra category in their Spanish perception mode, is correlated with their proficiency in identifying Spanish front vowels. We conclude that it is indeed multiple-category assimilation that is the cause of the problems that Dutch learners of Spanish have with the perception of Spanish front vowels.