Acquiring a language relies on distinguishing the sounds and learning mappings between meaning and phonetic forms. Yet, as shown in previous research on child language acquisition, the ability to discriminate between similar sounds does not guarantee success at learning words contrasted by those sounds. We investigated whether adults, in contrast to young infants, are able to attend to phonetic detail when learning similar words in a new language. We tested speakers of Korean and Mandarin to see whether they could use their native-language-specific perceptual biases in a word-learning task. Results revealed that participants were not able to fully capitalize on their perceptual abilities: only faster learners – as independently assessed by baseline trials – showed enhanced learning involving contrasts in phonetic dimensions informative in their native language. This suggests that attention to phonetic detail when learning words might only be possible for adults with better learning skills or higher motivation.