Previous research on event-related potentials (ERPs) on second language processing has revealed a great degree of plasticity in brain mechanisms of adult language learners. Studies with natural and artificial languages show that the N400 as well as the P600 component appear in learners after sufficient training. The present experiment tests if and which ERP components in response to syntactic and thematic processes generalize to unfamiliar lexical material in adult language learners. Learners of a miniature version of Japanese were presented with correct and incorrect sentences, half of which contained an unfamiliar word in the crucial sentence position. Incorrect sentences were either case-marking violations or word category violations. When all words were familiar, case-marking violations elicited a biphasic N400-P600 pattern and word category violations led to an early negativity that was followed by a P600. When the case violation occurred on an unfamiliar noun, only a P600 was seen. Word category violations that involved unknown verbs led to an early negativity and only to a reduced P600. The results suggest a high degree of nativelikeness for the learners during processing of familiar sentences. Unfamiliar words seem to entail additional processing costs and specifically lead to difficulties in the domain of case processing. (Contains 4 tables, 5 figures and 4 footnotes.)