The successful use of nanoparticulate ZnO in applications such as UV-screening agents or photocatalyst forthe destruction of chemicalwaste requires the development of techniques for controlling its photocatalyticactivity. In this study, we have investigated transition metal doping as a means of achieving this goal.Powders of ZnO, MnxZn1−xO, and CoxZn1−xOwere synthesised by a three-stage process consisting of highenergymechanical milling, heat treatment, andwashing. The photocatalytic activity of these powderswasevaluated using the spin-trapping technique with electron paramagnetic resonance spectroscopy. It wasfound that the photocatalytic activity of CoxZn1−xO progressively decreased with the doping level. Incontrast, the activity of MnxZn1−xO initially increased with doping up to a level of 2 mol% and thereafterdeclined. These results demonstrate that doping with transition metal oxides can be used to tailor thephotocatalytic properties of nanoparticulate ZnO.