Many simultaneous bilinguals exhibit loss or incomplete acquisition of their heritage language under conditions of exposure and use of the majority language (Silva-Corvalán, 1994, 2003; Polinsky, 1997; Toribio, 2001; Montrul, 2002). Recent work within discourse-functional (Silva-Corvalán 1994) and generative perspectives (Sorace, 2000; Montrul; 2002; Tsimpli, Sorace, Heycock, Filaci and Bouba, 2003, in press) suggests that while syntax proper is impervious to language loss or attrition, syntax-related interfaces like lexical-semantics and discourse-pragmatics are not. This study investigates argument expression in adult simultaneous bilinguals who are heritage speakers of Spanish, because in this language subjects, direct, and indirect objects are regulated by syntactic, pragmatic and semantic factors. It was hypothesized that if language loss affects interface areas of competence more than the purely syntactic domains, then Spanish heritage speakers should display robust knowledge of null subjects as well as object clitics, but variable behavior in the pragmatic distribution of null vs. overt subjects, the a preposition with animate direct objects, and cases of semantically based dative clitic-doubling. Results of an oral production task administered to 24 intermediate and advanced heritage speakers and 20 monolinguals confirmed the hypotheses. With the erosion of pragmatic and semantic features, the grammars of the intermediate proficiency Spanish heritage speakers appear to display morphosyntactic convergence with English in the expression of subject and object arguments.