Studies of the kinetics of anodic O2 evolution and of the adsorption behaviour of the kinetically involved intermediates, OH and O, in this process at the alpha and beta forms of PbO2 offers the possibility of examining how electrocatalysis in this reaction depends on the structure, and in particular the exposed surface structure, of these materials which otherwise have chemically identical compositions. Potential relaxation transients, following anodic current interruptions, and a.c. frequency response measurements have been used to derive information on the adsorption behaviour of the intermediates in relation to the steady-state polarization behaviour of the two "allotropic" forms of PbO2. Significant differences in the kinetic behaviour of 02 evolution and of the adsorbed intermediate species are found. The characterization of 02 evolution behaviour on lead dioxides is of practical relevance to the anodic processes that take place during overcharge of the positive plates of lead-acid batteries.