Unagreement patterns consist in a person feature mismatch between subject and verb that is nonetheless grammatical in Spanish. The processing of this type of construction gives new insights into the understanding of agreement processes during language comprehension. Here, we contrasted oscillatory brain activity triggered by Unagreement in different EEG bands with those triggered by Standard Agreement and Person Mismatch sentences. In Spectral Power analysis, a similar pattern for Unagreement to that elicited by Person Mismatch was found in lower beta and alpha bands, approximately around 600 ms. This suggests that fast oscillations reflect the evaluation of the feature match between two structurally related constituents (independently of its grammaticality). In an equivalent time window, the Unagreement condition behaves similarly to the Standard Agreement condition in the theta band, but different from the Person Mismatch condition that shows an enhanced spectral power. In addition, an enhanced phase synchrony in lower beta frequency around 600 ms is associated to standard agreement patterns, while beta-high frequencies at 800 ms characterize processing of Unagreement sentences. These results support the functional dissociation between the evaluation of the feature match between two structurally related constituents (independently of its grammaticality--in fast oscillatory bands) and integration of linguistic material in working memory (reflected by theta oscillations). Successful resolution of an agreement dependency is better reflected in phase synchronizations at beta band. (Contains 6 figures.)