This paper investigates morphosyntactic adaptation in second language (L2) sentence processing. In a pre-/posttest control group design, two experiments with intermediate to advanced German–English learners examine whether massed exposure to informative input leads to adaptation in L2 processing in that L2 readers come to integrate inflection in real-time comprehension. Experiment 1 on case marking shows that input causing prediction error and flagging the target parse leads to nativelike integration of case in the reanalysis of garden-path sentences. Experiment 2 shows partially nativelike processing of adverbial–verb tense mismatches after exposure to target input. Adaptation was selective to the experimental versus the control group in processing, yet it did not generalize to offline, explicit performance. We conclude that morphosyntactic adaptation constitutes an implicit learning mechanism in L2 processing, and we discuss its implications for models of L2 processing and acquisition.
