A recent study on syntactic influences on the mismatch negativity (MMN) reported preliminary evidence that processing or representational differences between verbs and nouns could be reflected in modulations of this early component of the event-related brain potential (ERP). However, this previous investigation was unable to separate the putative word category effects from possible influences of the direction of acoustic change on the MMN response. The present study disentangled these two effects by comparing the effects of the direction of acoustic change within and isolated from the lexical context that had previously been used to create the word-category contrast. Although the direction of acoustic change exerted a significant influence on the MMN amplitude, some of the temporal and topographic characteristics of the MMN and the overall pattern of the deviance-related ERP response proved to be specific to word-category effects. These findings provide the first reliable evidence that the MMN can serve as an index of word-category-specific processing and support the notion of early processing and representational differences between verbs and nouns.