This paper investigates the linguistic performance of a group of English-Spanish­ bilingual children (N = 44, age range: 10–14) with different degrees of language dominance in their use of the Spanish preposition ‘a’ using a completion task which measures the different conditions that relate to the distribution of this personal (Torrego 1998; Zagona 2002). The children’s performance was compared to a group of monolingual Spanish children of similar age (n = 10, age range: 12–15). All participants had also to complete an ethno-linguistic task which elicits data to address the impact of some external factors (e.g., language dominance and language choices) in language performance. Results revealed that bilingual children were less accurate than monolingual children in the use of the preposition ‘a’, but the two groups showed a similar pattern of errors regardless of their grade of bilingualism. More importantly, their linguistic performance does not seem to be related to any of the external factors included in this study. In line with previous research in the field, it is also claimed that phenomena involving the syntax-semantics interface are vulnerable in bilingual children irrespective of the grade of bilingualism and external factors (i.e., language dominance and language choice).