The water-oxidation catalytic activity of [RuII(damp)(bpy) (H2O)]2+ has been determined from manometric and mass spectroscopy studies. Mechanistic details of the catalytic cycle have been studied both experimentally and using DFT and CASSCF/CASPT2 calculations. Characterisation of this Ru(ii) complex and more highly oxidized catalytic intermediates has been accomplished through UV-vis and XAS spectroscopy, as well as through electrochemical techniques. Comparison of XAS spectra with CASSCF/CASPT2 calculations provides insight into the electronic structures of the more highly oxidized species, especially the degree to which oxidation occurs over both atoms of the Ru-O fragment. 18O-labelling experiments indicate that the O-O bond formation step proceeds via a water nucleophilic attack mechanism, and a detailed DFT analysis of the catalytic cycle predicts that step to be rate-determining and to take place for a formal Ru(v)O species. A number of alternative higher energy pathways have also been characterised in order to provide a more complete vision of the whole system. © The Royal Society of Chemistry 2012.