Two experiments investigated the time-course of semantic and syntactic processes in auditory language comprehension as well as their possible functional dependencies, using event-related brain potentials (ERPs). Participants listened to sentences which were either correct, semantically incorrect, syntactically incorrect, or both semantically and syntactically incorrect. In experiment 1, participants judged the overall correctness of these sentences. The semantic violation elicited an N400 whereas the syntactic phrase structure violation elicited an early anterior negativity followed by a P600. Sentences in which the critical element violated both semantic and syntactic constraints elicited the same pattern of ERPs as the syntactic violation alone, not evoking an N400. In experiment 2, participants judged the same sentences for semantic coherence, required to ignore syntactic violations. Again, an early anterior negativity was elicited for those sentences containing phrase-structure errors. In contrast to experiment 1, however, combined violations elicited both an early negativity and an N400. Together, the results suggest that the N400 associated with semantic aspects of sentence comprehension reflects controlled processes whereas initial parsing processes associated with the early anterior negativity are independent of semantic constraints and task requirements.