Music consists of perceptually discrete elements that are organized according to syntactic regularities. Violations of these regularities typically elicit two ERP components: the ERAN and the N5. In several studies, we tried to disentangle music-syntactic and psychoacoustic influences on the underlying cognitive processes. In a first study, we compared the electrophysiological response to chord sequences that differed in the music-syntactic regularity, but were similar with regard to sensory factors (such as pitch commonality, pitch repetition, and roughness). We showed that ERAN and N5 were elicited, irrespective of these similarities, indicating that these ERP components are an index of music-syntactic processing. A second study evaluated effects of long-term exposure on the processing of music-syntactic irregularities. The ERAN amplitude declined over the course of an experimental session (about 120 min), suggesting that cognitive representations of musical regularities can change implicitly, in response to the repeated presentation of unexpected, irregular harmonies. A third study explored, whether the ERAN is actually elicited by irregular chords (containing several voices) or by a deviance in the (most prominent) upper voice. We demonstrated two different patterns of neurophysiological responses to these two irregularities, strengthening the assumption that two different cognitive mechanisms are involved in the processing of irregularities in chord sequences, and melodies, respectively. Finally, we tried to follow up the development of the neural correlates of music-syntactic processing. We found an increase between the second and the fifth followed by a decline between the fifth and the eleventh year of age for the ERAN. Almost the same course of development was observed for the N5 (which was however not present in the two year olds). However, the observed differences between the age groups were not significant, indicating a quite high stability in the brain responses to music-syntactic violations.