The kinetics of the oxygen evolution reaction (oer) has been investigated in commercial NiMH batteries. The partial oxygen evolution currents have been determined during partial charging by measuring the charge distribution between the main electrochemical charge transfer reaction at the Ni-electrode and the parasitic oxygen evolution reaction. This enabled us to accurately determine the battery State-of-Charge during the entire charging process and to deduce information about the oxygen evolution reaction kinetics as a function of temperature. In addition, the oer kinetics has also been studied under continuous overcharging conditions. Substantial kinetic differences were observed. This has been attributed to the phase modification of nickel-oxyhydroxide induced by prolonged overcharging. The kinetic parameters of the oxygen evolution reaction have been determined, the mechanisms are discussed in detail and the relevant rate equations are derived to enable NiMH battery modelling.