Error patterns in vocabulary learning data were used as a window into the mechanisms that underlie vocabulary learning performance in bilinguals vs. monolinguals. English--Spanish bilinguals (n = 18) and English-speaking monolinguals (n = 18) were taught novel vocabulary items in association with English translations. At testing, participants produced English translations for the newly learned words. Findings revealed broad learning advantages in the bilingual data. Moreover, bilinguals made proportionately more deep (within-category) semantic errors than monolinguals when tested immediately after learning. We interpret these data to suggest that bilingual learners may encode the information associated with the novel words to a deeper semantic level than monolinguals.