The aim of the present study is to examine clitic substitution–the extension of the use of a particular clitic in the paradigm–by Catalan-speaking children, whatever their dominant language, who variably acquire the oblique clitics hi and en. We analyse the majority of these cases of substitution within a theory of feature underspecification and feature geometry. One type of clitic substitution cannot, however, be accounted for with our proposed feature geometry. In this case, we propose that this unexpected outcome is due to a constraint on morphological complexity which results in recourse to more salient forms.