Despite exhaustive spectroscopic investigations on the CoPi oxygen-evolving catalyst over the past several years, little is known about the surface cobalt sites and intermediates in direct contact with water that are responsible for the actual catalysis. Many studies thus far have been limited to ex situ characterizations or bulk film measurements, often in the absence of solvent. Here we describe an investigation of the CoPi catalyst by surface interrogation scanning electrochemical microscopy (SI-SECM). This method should allow us to selectively study surface atoms separately from the bulk in a solvent-filled environment. By SI-SECM, independent titrations of surface Co-III and CoIV were performed, yielding a direct measurement of the surface active-site density of a CoPi electrode (11 Co/nm(2)). The pseudo-first-order reaction rate constants of Co-III and CoIV with water were determined to be 0.19 and >2 s(-1), respectively, through time-dependent titration measurements.