Understanding the factors underlying variation in attentional state is critical in a number of domains. Here, we investigate the relation between time on task and mind wandering (i.e., a state of decoupled attention) in the context of a lecture. Lectures are the primary means of knowledge transmission in post secondary education rendering an understanding of attentional variations in lectures a pressing practical concern. We report two experiments wherein participants watched a video recorded lecture either alone (Experiment 1) or in a classroom context (Experiment 2). Participants responded to mind wandering probes at various times in the lecture in an effort to track variations in mind wandering over time. In addition, following the lecture, memory for the lecture material was tested. Results demonstrate that in a lecture mind wandering increases with time on task and memory for the lecture material decreases. In addition, there was a significant relation between mind wandering and memory for lecture material. Theoretical and practical applications of the present results are discussed. Copyright © 2011 John Wiley & Sons, Ltd.