This study investigates whether adult second language learners' online processing of "wh"-dependencies is constrained by island constraints on movement. Proficiency-matched Spanish and Korean learners of English completed a grammaticality judgment task and a stop-making-sense task designed to examine their knowledge of the relative clause island constraint and their sensitivity to this constraint in online "wh"-dependency formation. The results showed that both learner groups have knowledge of the island constraint in English. However, unlike the Spanish speakers, who immediately applied the constraint to prevent the formation of an ungrammatical "wh"-dependency, the Korean speakers showed evidence of temporarily entertaining an ungrammatical dependency. These findings suggest that the properties of the native language influence the online processing of L2 sentences.