While much research shows that early sensory and attentional processing is affected by mind wandering, the effect of mind wandering on deep (i.e., semantic) processing is relatively unexplored. To investigate this relation, we recorded event-related potentials (ERPs) as participants studied English-Spanish word pairs, one at a time, while being intermittently probed for whether they were 'on task' or 'mind wandering.' Both perceptual processing, indexed by the P2 component, and deep processing, indexed by a late, sustained slow wave maximal at parietal electrodes, was attenuated during periods preceding participants' mind wandering reports. The pattern when participants were on-task, rather than mind-wandering, is similar to the subsequent memory or difference in memory (Dm) effect. These results support previous findings of sensory attenuation during mind wandering, and extend them to a long-duration slow wave by suggesting that the deeper and more sustained levels of processing are also disrupted. [This paper was published in "Neuroreport" v29 n5 p380-384 Mar 2018.]