Sentence interpretation was examined with event-related brain potentials (ERPs). The ERPs were recorded while participants listened to French sentences containing a subject-modifying relative clause (SRC). These were either correct, semantically incorrect, syntactically incorrect, or doubly (syntactically and semantically) incorrect. The semantic anomaly realized as a selectional restriction violation was associated with an N400. The syntactic anomaly realized as a phrase structure violation in the SRC elicited a frontal negativity between 150 and 600 ms. This negativity was more pronounced in the left than in the right hemisphere in the early time window (150-300 ms). In a later time window (300-600 ms), it was more broadly distributed including anterior and posterior regions, but with a maximum over the anterior recording sites. Finally, a centro-parietal late positivity (P600) was found between 600 and 1000 ms. While syntactic and semantic information in the double violation condition did not interact between 150 and 300 ms, they did interact between 300 and 600 ms. This finding supports serial models of sentence processing that postulate an initial autonomous stage of phrase structure building and a late stage of interaction.