Nickel-iron mixed metal oxyhydroxides have attracted significant attention as an oxygen evolution reaction (OER) catalyst for solar fuel renewable energy applications. Here, we performed surface-selective and time-dependent redox titrations to directly measure the surface OER kinetics of Ni-IV and Fe-IV in NiOOH, FeOOH, and Ni1-xFexOOH (0 < x < 0.27) electrodes. Most importantly, two types of surface sites exhibiting "fast" and "slow" kinetics were found, where the fraction of "fast" sites in Ni1-xFexOOH matched the iron atom content in the film. This finding provides experimental support to the theory-proposed model of active sites in Ni1-xFexOOH. The OER rate constant of the "fast" site was 1.70 s(-1) per atom.