This study examined auditory ERP responses to syntactic phrase structure violations occurring either in sentences containing regular words or in sentences in which content words had been replaced by pseudowords while retaining morphological markers (so-called jabberwocky sentences). Syntactic violations were found to elicit an early anterior negativity followed by a P600 for both types of sentences, suggesting that the syntactic processes in question are independent of the presence of lexical-semantic information. In syntactically correct sentences, content words in regular sentences elicited an N400 component while their pseudoword place-holders in jabberwocky sentences did not. By contrast, in syntactically incorrect sentences neither sentence type showed an N400 for the word creating the syntactic violation, indicating that the detection of a syntactic error at an early stage blocks semantic integration processes in regular sentences. We discuss these results and findings from related studies in the light of a timing hypothesis of syntactic and semantic information processing and propose that syntactic information extracted particularly early can affect semantic processes while syntactic information extracted relatively late cannot.