(Figure Presented) A thorough kinetics investigation of the Ru-Hbpp water oxidation catalyst has been carried out at temperatures in the range 10-40 °C. Four oxidative electron-transfer processes that take the catalyst from its initial II,II oxidation state up to the formal IV,IV oxidation state were kinetically characterized and the corresponding activation parameters determined. Once the IV,IV oxidation state is reached, two additional slower kinetic processes take place, corresponding to the formation of an intermediate that finally evolves oxygen and regenerates the initial Ru-Hbpp catalyst. These two kinetic processes were also fully characterized with respect to the evaluation of their rate constants and activation parameters. Furthermore, 18O labeling experiments were performed with different degrees of labeled catalyst and solvent, and the 16O2/ 16O18O/18O2 isotopic distribution of the generated molecular oxygen was calculated. These results clearly point to the existence of a single intramolecular reaction pathway for the formation of the oxygen-oxygen bond in the case of the Ru-Hbpp catalyst. Copyright © 2009 American Chemical Society.