This paper explores the code-switching behavior of second language (L2) bilinguals as a lens into the development of their L2 linguistic systems. Specifically, it investigates the acceptability judgments of L1-English L2-Spanish bilinguals on intra-sentential code-switching, comparing those judgments to a group of Spanish–English bilinguals who acquired both languages as an L1. The particular issues of proficiency and bilingual language behavior are analyzed, testing whether either factor has an effect on L2 code-switching intuitions. The results suggest that both proficiency and bilingual language behavior are relevant. L2 bilinguals with an intermediate/advanced proficiency level of Spanish were more likely to align with 2L1 bilinguals with regard to code-switching judgments, as were L2 bilinguals who reported prior experience with language mixing. L2 bilinguals with lower proficiency in Spanish, as well as those who reported never engaging in code-switching, however, were more likely to diverge from the 2L1 bilinguals in their judgments.