Executive functioning deficits are reported in ecstasy users. However research into mental set switching has been equivocal, with behavioral studies suggesting the function is preserved. The current study sought to address the issue of switching deficits in ecstasy users by combining behavioral performance with electrophysiological correlates (electroencephalography; EEG). Twenty ecstasy polydrug users, 20 nonecstasy polydrug users, and 20 drug naive controls were recruited. Participants completed questionnaires about their drug use, sleep quality, fluid intelligence, and current mood state. Each participant completed a mental set switching task (the number-letter task) while EEG measures were recorded. Analysis of variance (ANOVA) revealed no between-group differences on performance of the task; however a regression suggested that ecstasy use was a significant predictor for performance, after controlling for cannabis use. Mixed ANOVA revealed a significant effect of group on the P3, with significant differences between both drug groups and naives. There was also an interaction between electrode and group on the P2 component, with ecstasy users differing from both other groups. On the P3 component the results suggest a reduction in positivity at parieto-occipital electrodes for drug users compared to controls. Furthermore a significant increase in negativity in ecstasy users compared to control groups could be observed in several occipito-parietal electrodes at an N2 component as well as observable atypicalities in early processing (P2) displayed by ecstasy users and polydrug controls. The present study provides evidence of atypical processing of attentional shifting in ecstasy and polydrug users. Deficits in this executive function could reflect cognitive inflexibility and paucity of rapid behavioral adjustment, which may be problematic in real world situations.