Previous studies have shown that increases in pupil size are correlated with increasing cognitive processing demands. Our aim was to confirm whether these findings could be replicated with new portable and less obtrusive eye-tracking technology. We assessed the percentage change of pupillary diameter from baseline as eight subjects completed a series of randomly ordered arithmetic problems of varying difficulty. The mean peak pupil diameter expressed as a percentage change from baseline was significantly greater when answering difficult questions compared to easier questions. Moreover, the time to reach peak pupillary diameter occurred significantly faster when participants answered easier questions compared to more difficult questions. Finally, there was a significant difference when all groups were compared to control. This experiment confirms findings of previous studies that show that pupillary size is related to cognitive processing demands. It also demonstrates that mobile eye-trackers can be used to reliably gather this type of data. Furthermore, this experiment provides the basis for future studies using eye-tracking technology in new environments, for example in the study of expertise and performance in medical crisis situations.