We review the basic principles of ultrafast charge delocalization phenomena in aqueous electrolytes following core-level photoionization and resonant core-level excitations, and describe how these processes can be probed using Auger electron spectroscopy. The use of the core-hole clock method to extract dynamical information from Auger spectra is described. By a number of examples we demonstrate how the efficiency for ultrafast charge transfer phenomena on the low femtosecond timescale is determined not only by factors such as charge, polarizability and solvated radius of the ionic solutes but also by the dominant interactions of the ionized site with its surroundings in the intermediate state. (C) 2012 Elsevier B.V. All rights reserved.