Possessive determiners (PDs) "his" and "her" are challenging for L2 learners to acquire, and this difficulty has been attributed to several factors, including negative L1 transfer effects (White et al., 2007). What researchers have not yet considered is how PDs are acquired by learners whose L1 predicts positive transfer effects. To address this question, the present study investigated the PD interlanguage of Brazilian Portuguese (BP) speakers, whose L1 has a PD system that is similar to English. It also considered the effects of crosslinguistic awareness on performance of this linguistic feature, as awareness between and across languages has been shown to support positive transfer (e.g., Gibson & Hufeisen, 2008). Two written and two oral tasks were used to measure performance, and a stimulated recall task was used to measure awareness. The results showed that BP-speakers exhibited advantages in their acquisition of "his" and "her" in comparison to previously studied L1 groups, and that learners who verbalised awareness of a crosslinguistic PD rule outperformed those who did not on two of three PD tasks. These findings contribute to the research suggesting that building crosslinguistic awareness of L1/L2 similarities could be an effective approach for supporting language learning.