With increasing age, the electrophysiological event-related potentials P3a/P3b tend to get a more frontal maximum. The cognitive significance of this so-called frontal shift is not known, but hypotheses have focused on changes in the integrity of the frontal lobes. The aim of the present study was to test how the thickness of the cerebral cortex is related to the frontal shift. Well screened elderly participants went through a visual three-stimuli oddball-task, a battery of neuropsychological tests and magnetic resonance imaging scans. It was found that participants with frontocentral maxima had a thicker cerebral cortex in distinct areas than participants with parietal maxima, both for P3a and for P3b, while the parietal P3b participants had a thicker cortex in the anterior cingulate. This is the first study to demonstrate that age-dependent changes in the scalp distribution of electrophysiological activity are related to differences in thickness of the cerebral cortex.