The rates of different types of reactions for Model 1 and Model 2 are shown in Figure , including proton loss, hole trapping, and thermal reactions such as O-O coupling, OH radical transfer and O2 release. The rates of these processes track the overall photo-OER rate as shown by the hole photocurrent in Figure . Proton loss to form a negatively charged product is the fastest process under all conditions, followed by hole trapping by Ti-OH -in pathway I. All other reactions have nearly equal rates and are uniformly slower than proton loss. By themselves, the rates do not provide clear information on which reaction steps most strongly influence the overall photo-OER rate or why the photo-OER rate saturates with light intensity. Consideration of the populations of intermediates in Figure provides insight. They show that the intermediates that build up in pathway I are ones that need a hole to react at low light intensity, and ones that must undergo a thermal reaction at high light intensity e.g. TiO --TiO -coupling and loss of OH from Ti-OH. Adsorbed water becomes an important surface species, indicating that heterolytic dissociation of it to form a proton and Ti-OH -is also relatively slow at high light intensity.