An experiment investigated adult language learners' ability to develop fully integrated cognitive representations of a difficult second language (L2) morphosyntactic feature: the Korean honorific verbal affix (u)si Native speaker (NS) and nonnative speaker (NNS) latencies during a word-by-word self-paced reading comprehension task were measured. Wilcoxon signed ranks tests indicated that NSs exhibited sensitivity to grammaticality within contexts in which the honorific verbal affix (u)si was present; they did not display such sensitivity in more ambiguous contexts in which the verbal honorific marking was absent. At the second and third positions following the critical point at which the grammaticality of the sentence was evident, NS reaction times for the ungrammatical conditions were significantly longer and the effect sizes were large, z = -4.94, p < .001, r = -0.87; z = -3.89, p < .001, r = -0.67. An item analysis yielded similar results. NNSs, on the other hand, did not demonstrate sensitivity to grammaticality at any of the word positions in participant or item analyses. The experiment suggests that even advanced learners who clearly demonstrate explicit knowledge of the Korean affix (u)si lack integrated morphosyntactic knowledge.