<p>High cognitive reserve (CR) is associated with a set of lifestyle factors such as high education level and occupational attainment. Studies suggest that CR is linked to enhanced executive control functions. Nevertheless, it is unclear what functional neural activity patterns are linked to high CR and to what extent they are steady across different cognitive states. The aim of the present study was to investigate electrophysiological differences between low CR (LCR) and high CR (HCR) at rest and during the performance of two cognitive control tasks with different difficulty level. Thus, 67 older adults performed an experimental session consisting of two spatial stimulus–response compatibility (SRC) tasks (i.e., the Simon task, and the more demanding spatial Stroop task), and a resting state during an electroencephalogram (EEG) recording. Cognitive control was better in the HCR than in the LCR group, as revealed by higher accuracy in the HCR compared to the LCR group in incongruent conditions of the tasks. Moreover, event-related brain potentials (ERPs) showed earlier latencies (P300) in HCR than LCR and time/frequency analysis showed higher alpha activity in HCR than LCR during the performance of the tasks, while no differences were observed at rest within any of the analyzed frequency bands. Higher P300 frontalization was observed in LCR than HCR in the more difficult task but it was not related to frontalization within a specific frequency band. Overall, results showed that electrophysiological differences between high and low CR depend on the cognitive state, with greater CR-related differences emerging at increased task demands.</p>