The intermediates generated in the process of oxygen activation at metal centers participate in hydrogen and oxygen atom transfer, electron transfer, substitution, acid-base chemistry, and free radical chemistry. The reactivity and intrinsic lifetimes of such intermediates in aqueous solutions are a strong function of pH as the metal-oxygen interaction adds an extra dimension to the already complex pH dependence of O 2 reduction. Acid-base chemistry at "nonparticipating" ligands plays a major role in the kinetics and mechanisms, and can even determine the outcome of some reactions. Superoxometal complexes are subject to homolytic metal-oxygen bond cleavage in acidic solutions, but decompose by heterolysis at higher pH. Reactions of halides by hydroperoxo and peroxo complexes proceed through two major channels -oxidation of halide ions, and catalysis of H 2 O 2 disproportionation -in close resemblance to enzymes haloperoxidases. The combination of the thermodynamics of electron transfer and protonation equilibria make transition metal hydroperoxo complexes both better oxidants and better reductants than the parent H 2 O 2 in 2-electron reactions.