Mind-wandering is both pervasive and detrimental to task performance. As such, identifying covert physiological measures that are associated with this off-task state could inform theories of mind-wandering and lead to interventions that improve task focus. Although previous work suggests that pupil dilation (PD) may vary between on- and off-task states, no studies have examined whether PD systematically varies within a subject as they report becoming disengaged from a task; a key step in developing useful mind-wandering prediction algorithms. In the present study, PD was measured while participants advanced through a passage one word at a time. Spontaneous mind-wandering was assessed during reading using standard thought probe methodology. Results revealed higher PD prior to off-task compared to on-task reading. This newly discovered relationship between momentary fluctuations of attention and PD offers promise for future innovations that use these systematic changes in PD to predict and better control mind-wandering.