In this study, we presented sentences either ending with high or low probability cloze words or semantically incongruent words to investigate the influence of L2 proficiency on electrophysiological correlates of semantic processing in bilinguals. Event-related potentials (ERPs) as well as the oscillatory dynamics of the EEG signal, specifically, frequency power changes expressed as event-related (de)synchronization (ERD/S), were analyzed. Replicating earlier results, we found an N400 on semantically incongruent words, as well as on low cloze probability words. For the bilinguals investigated in the present study, N400 latency in the low cloze probability condition was found to be modulated by L2 proficiency, indicating that L2 proficiency in our sample might have influenced the speed of semantic integration. Relative Theta power increased for all three word conditions, but no influence of proficiency was observed. Different from the ERP results, we found a stronger increase in theta power for low cloze probability words than for incongruent and high cloze probability words, especially over temporo-parietal brain areas. The spatial distribution of the theta ERD/S results also differed from the N400 topography. Whereas the N400 showed a typical topography with a maximum over temporo-posterior electrode positions, the theta ERD/S topography was maximal for high and low cloze probability words over left central-posterior electrode positions. These findings show that the ERP and ERD/S results are sensitive to semantic processing. The different pattern of the ERD/S results compared to the ERP results, functionally and with regard to topography, suggests that the ERD/S reflects a different aspect or stage of semantic processing, possibly the successful conceptualization of a sentence.