A novel, highly efficient, and stable water oxidation catalyst was prepared by a pH-controlled adsorption of Co(II) on similar to 10 nm diameter silica nanoparticles. A lower limit of similar to 300 s(-1) per cobalt atom for the catalyst turnover frequency in oxygen evolution was estimated, which attests to a very high catalytic activity. Electron microscopy revealed that cobalt is adsorbed on the SiO2 nanoparticle surfaces as small (1-2 nm) clusters of Co(OH)(2). This catalyst is optically transparent over the entire UV-vis range and is thus suitable for mechanistic investigations by time-resolved spectroscopic techniques.