Auger-electron spectroscopy is applied to hydrogen peroxide aqueous solution to identify ultrafast electronic relaxation processes, specifically those involving a proton transfer between core-ionized hydrogen peroxide and solvating water molecules (proton transfer mediated-charge separation, PTM-CS). Such processes yield dications where the two positive charges resulting from the Auger decay are delocalized over the two molecules. These species contribute to the high-energy tail of the Auger-electron spectrum as do also species resulting from charge delocalization in the ground-state geometry. However, the immediate and secondary transient species are different for ground-state and proton-transferred structures. Here we show that it is possible to experimentally distinguish the species by studying the H2O2/D2O2 isotope effect on the Auger spectra. To interpret the measured Auger-electron spectra, we complement the experiment with ab initio based dynamical calculations.