We introduce electrochemical imaging and nano-resolved measurements of catalytic intermediates on operating SrTiO3 photoelectrodes. Spatially resolved redox titrations of photogenerated reactive oxygen species (ROS) were used to profile changes in ROS coverage and reactivity at pristine and ion-milled defective areas on n-doped (100) SrTiO3. Adsorbed ROS reached a potential-dependent limiting coverage of 0.1 monolayer and did not differ significantly between milled and pristine areas. However, the reaction kinetics between a solutionphase mediator and adsorbed ROS were found to be significantly decreased over ion-milled areas. Using a nanoelectrode, we resolved k(s,i) values of 5 and 300 m(3)/s. mol for these bimolecular reactions at defective and pristine sites, respectively. Ion-milled areas also showed significantly decreased activity toward photo-oxidations, providing evidence that photogenerated ROS mediate fast charge-transfer reactions with solution-phase species at the semiconductor electrolyte interface. Our results provide spatially resolved direct evidence of the impact of surface defects on the performance of photoelectrochemical systems. Scanning electrochemical microscopy offers a powerful method for evaluating the reactivity of an operating electrochemical interface by using redox titrations that detected as few as 30 attomoles of adsorbed ROS.