At high concentrations, nitrate ion alters the dynamics of ruthenium "blue dimer"-catalyzed water oxidation by Ce(IV) such that the oxidation rate is enhanced and a unique reaction intermediate accumulates. This intermediate is characterized by distinct EPR, optical, and resonance Raman (RR) spectra, with the appearance in the latter of a new oxygen isotope-sensitive band. Both Ce(IV) and nitrate are required to generate this intermediate, which suggests ceric-nitrate complexes as the causative agents. Use of O-18-labeled and N-15-labeled materials has established that (1) the new RR band is not an O-O stretching mode (for example, as might be associated with a peroxo species) but involves the O atom coordinated to a Ru center, and (2) the O-2 product does not contain an O atom derived from nitrate, eliminating several plausible pathways involving O-atom transfer to oxidized dimer. Although these results are surprising, similar phenomena have been reported for water oxidation catalyzed by monomeric Ru complexes. The dramatic effects observed for the "blue dimer" make it an ideal candidate for further study.