Previous research has shown that first language (L1) American English speakers discriminate the Spanish /[alveolar tap]/-/r/ and /[alveolar tap]/-/t/ contrasts significantly better than the /[alveolar tap]/-/d/ contrast, regardless of their proficiency level in Spanish (Rose, 2010a). Therefore, the current study follows the framework of the Perceptual Assimilation Model (e.g., Best, 1995) and its application to the L2 context (PAM-L2; Best & Tyler, 2007) to ascertain why L1 English speakers discriminate certain L2 Spanish contrasts before others. Thirteen L1 English speakers with little (n = 6) or no Spanish experience (n = 7) completed a cross-language mapping task. Results indicate that native English speakers experience difficulty discriminating the /[alveolar tap]/-/d/ contrast because they categorize the Spanish /[alveolar tap]/ and /d/ to similar phonemic categories in English. Results further suggest that both L2 Spanish experience and lexical context affect how L2 consonants are categorized in an L1. (Contains 4 tables.)