Event-related potentials and fields have been extensively explored in psycholinguistic research. However, relevant information might also be contained in induced oscillatory brain responses. We used magnetoencephalograhy (MEG) to explore oscillatory responses elicited by semantically incongruent words in a classical sentence comprehension paradigm. Sentences in which the last word was either semantically congruent or incongruent with respect to the sentence context were presented auditorily. Consistent with previous studies a stronger N400m component was observed over left temporal areas in response to incongruent compared to congruent sentence endings. At the same time, the analysis of oscillatory activity showed a larger beta power decrease (16-19 Hz) for the incongruent than congruent condition in the N400m time window (200-700ms), also over the left temporal area. The relationship between the beta decrease and the N400m was confirmed by a linear regression analysis. Moreover, using a beamforming approach we localized the sources of the beta decrease to the left prefrontal cortex (BA47). We propose that the beta oscillation reflects the engagement of brain networks. A lower beta power indicates a higher engagement for information processing. When the input is highly predictable (congruent condition), a lower beta power in the pre-stimulus interval predicts a better performance (smaller N400m); while a low predictability (incongruent condition) of the input shows a relationship between the N400m and the beta power in the post-stimulus interval, which indicates the engagement of the brain networks for integrating the unexpected information. This 'engagement'hypothesis is also compatible with reported beta effects in other cognitive domains.