This study examines the second language (L2) development of variable /s/-weakening in the spontaneous speech of L2 learners of Spanish who studied abroad in either Dominican Republic, where /s/-weakening is widespread, or central Spain, where /s/-weakening is much less common. Learners' realizations of /s/ were coded impressionistically and acoustically by measuring voicing, center of gravity, and duration. The results show that regardless of the study abroad location, students did not change the amount of sibilance they produced over time. However, they became more nativelike with respect to /s/-voicing and duration. Additionally, whereas some linguistic factors were found to significantly constrain /s/-weakening across groups, learners did not gain sensitivity to all factors that constrain native-speaker /s/-weakening. Findings suggest that exposure to /s/-weakening during a semester abroad is insufficient for learners to adopt this sociolinguistic variable and other social and cognitive factors likely mitigate its integration into the L2 learners' phonological systems.