Failure to produce simple past morphology consistently well in adult SLA has been observed in L2 English, especially of native speakers of Chinese. On the one hand, it has been argued that this results from an L2 inability to acquire new functional features (Hawkins and Liszka 2003), while on the other it has been argued that this behavior can be better accounted for via a mapping problem stemming from a dissociation between formal syntactic features and their corresponding morpho-phonological forms (e.g., Lardiere 2000). A challenge to determining which of these accounts is more explanatory is the fact that for this property there are at least two confounded possibilities: past tense is not grammaticalized in Chinese, and/or coda clusters (frequent in English regular past forms) are disallowed in Chinese. This paper attempts to tease apart syntactic and phonological transfer in adult SLA by looking at a group of L1 Spanish/L2 English for the same properties, since in Spanish past is grammaticalized but complex codas are heavily restricted. It is concluded that L1-transferred phonotactic constraints are pervasive in adult L2 acquisition, and thus might better explain what has been observed for Chinese natives.