As shown in Figure , there are two pathways that can produce oxygen. Using the simulated photocurrents for each pathway, we can distinguish their contributions to the total photocurrent in Models 1 and 2 as shown in Figure . In Model of the total photocurrent at high light intensity. This is a consequence of requiring that hole trapping involves adjacent sites, so that the coupling between steps 2 and 8 looks much more like a unimolecular rearrangement than transfer of a free OH. We can compare this finding to the microkinetics simulation results of Wang et al. using the same photo-OER mechanism and assuming only mobile surface holes, similar to our Model 1 but without including the TiO2 photophysics. While the overall trend in reaction rate with hole concentration is similar to ours (Figure (a)) including a transition to a plateau in rate around 10 -4 h + nm -2 , their predictions regarding the balance between pathways I and II are different. They predict that the competition between the two parallel pathways depends on the hole concentration in TiO2, with pathway I dominating at low hole concentrations, and pathway II occurring at a similar rate to pathway I at high hole concentrations. Looking at Figures and, it is clear that the surface concentration of holes is nearly constant as a function of light intensity at a value of about 10 -4 h + nm -2 , as opposed to the total concentration of holes in the system which does vary. This may be due to the fact that they only studied the effect of surface chemical reactions on OER and did not consider the effect of carrier transport in the bulk. However, it is clear that kinetic modeling of photodriven systems must be done with care because the influence of charge carrier transport may not always be neglected. We predict that under Model 1 pathway I is strongly dominant under all light intensities, in good agreement with the value of about 100:1 found by Wang et al. for our predicted surface hole concentration. In the work on anatase (101) surface, Wang et al.