In this paper we explore the potential of analyzing pupil diameter measurements obtained using a desktop-mounted Eye Gaze Tracking (EGT) instrument to identify changes in the affective state of a computer user. In our experiment we induced intervals of relaxed and stressed affective states by asking the computer user to respond to sequences of congruent and incongruent Stroop word presentations. The recorded pupil diameter values verify our initial expectations by showing an increase in pupil diameter mean value as the subject transitions form a congruent Stroop sequence or segment to an incongruent Stroop segment. This mean pupil diameter value increase was found in all but one of the 96 transitions studied from 32 subjects. The statistical significance of these mean value increases was studied by means of a t-test for the comparison of means. All the pairs of mean pupil diameter values were found to be significantly different at a 0.05 significance level (p <0.05). This seems to indicate that the real-time measurement of pupil diameter of the computer user holds a strong promise to become a non-intrusive way to make the computer aware of changes in the affective status of the user. However, it is clear that several implementation issues still must be resolved before this approach can be practical for use in the context of ordinary human-computer interaction, where, for example, the environmental illumination levels are not controlled, as they were during our experiments.