This research provides a usage-based account of the phonological reduction of syllable and wordfinal /s/ in the bilingual Spanish of the Uruguayan-Brazilian border. As with monolingual dialects of Uruguayan Spanish, patterns of aspiration and deletion seem to be conditioned by lexical frequency, with words of high frequency demonstrating higher rates of reduction than words of low frequency. Unlike these dialects, however, the deletion of /s/ in border varieties may be conditioned by Portuguese morphological patterns in which plural /s/ is deleted for head nouns and adjectives but retained for determiners. This possibility is explored by examining the linear and relative positions of plural NP constituents as well as the plural marking patterns of preceding constituents. Multivariate analysis shows that /s/ deletion is statistically probable in highly frequent words whereas aspiration is favored among words of low frequency. The latter finding may be the result of a social distribution of aspirated variants among more upwardly mobile residents. The use of both variants is conditioned by the morphological patterns of Portuguese, in which plural marking is carried out through the use of overt articulations for plural determiners and first position NP constituents while plural markers are deleted for nouns and adjectives, especially for second and third position constituents. These results provide further evidence of the uniqueness of bilingual dialects and support the claim that bilinguals have more expansive linguistic repertoires than monolinguals.