The existence of a critical period is a topic of controversal discussion in the second language acquisition research (e.g. Birdsong, 1999). Studies using brain imaging techniques as well as electrophysiological studies provide inhomogeneous findings of the brain organization of first and second language (e.g. Indefrey, 2006). The most discussed factors that influence the bilingual brain organization are the age of acquisition and the attained language proficiency (e.g. Abutalebi 2001; Müller 2003). In the present study, the EEG technique was used to investigate the extent to which the two factors - age of acquisition and language proficiency - influence the brain's responses to online comprehension processes of first and second language in Korean-German bilinguals who were exposed to second language either since their birth or from puberty onwards and who had different language proficiency in second language. The results revealed that the ERP response to semantic violation was affected by the attained language proficiency. In contrast, the ERP response to syntactic violation was influenced by both age of acquisition and language proficiency. Besides, a task-specific P600 pattern was observed in relation to language proficiency. These findings suggest that both age of acquisition and language proficiency affect the syntactic system more than the semantic system. The task-specific P600 pattern, which was shown only by the less proficient bilingual group, may be interpreted in two ways. On the one hand, the syntactic complexity plays an additional important role in the ERP correlate of syntactic processing. On the other hand, the effect of syntactic complexity occurs in relation to the effect of language proficiency.