To determine the effectiveness of Stroop protocols (Stroop, 1935) as mentally fatiguing interference tasks, subjective measures such as visual-analog-scales (VAS) are often used. There is significant inter-individual variability associated with these types of scales however (Dittrich et al., 2007), and the absence of direct assessments may lead to overestimation of the degree to which Stroop truly induces mental fatigue. For example, we suggest that, not only should tasks sensitive to mental fatigue reveal performance decrements following a Stroop intervention, performance on the Stroop task itself should deteriorate as well. Participants completed a Raven's Progressive Matrices test, shown as sensitive to fatigue (Linde & Bergströme, 1992) and were then randomly assigned into either Stroop or control (watch a documentary) groups. Following these interventions, participants completed a second Raven's test and recorded perceived levels of mental fatigue on VAS every two minutes. As expected, VAS scores increased with time suggesting increasing subjective levels of mental fatigue. Of greater interest was that, although performance on Raven's declined predictably with difficulty, it did so at the same rate for both groups. Furthermore, there was no evidence that performance on the Stroop task itself (in either RT or error data) deteriorated as the study progressed. Thus, although participants in the Stroop condition self-reported typically observed increases in VAS measures of mental fatigue, there was no evidence to suggest that Stroop: a) negatively influenced performance on a direct measure of executive function or b) that Stroop performance deteriorated as mental fatigue (suggested by VAS scores) accumulated.