In this article, we report the results of a self-paced reading experiment designed to investigate the question of whether or not advanced French and German learners of English as a second language (L2) are sensitive to tense/aspect mismatches between a fronted temporal adverbial and the inflected verb that follows (e.g. *"Last week, James has gone swimming every day") in their on-line comprehension. The L2 learners were equally able to distinguish correctly the past simple from the present perfect as measured by a traditional cloze test production task. They were also both able to assess the mismatch items as less acceptable than the match items in an off-line judgment task. Using a self-paced reading task, we investigated whether they could access this knowledge during real-time processing. Despite performing similarly in the explicit tasks, the two learner groups processed the experimental items differently from each other in real time. On-line, only the French L2 learners were sensitive to the mismatch conditions in both the past simple and the present perfect contexts, whereas the German L2 learners did not show a processing cost at all for either mismatch type. We suggest that the performance differences between the L2 groups can be explained by influences from the learners' first language (L1): namely, only those whose L1 has grammaticized aspect (French) were sensitive to the tense/aspect violations on-line, and thus could be argued to have implicit knowledge of English tense/aspect distinctions.