The effects of solvent pH and deuteration on the transient kinetics of the key intermediates of the dioxygen activation process catalyzed by the soluble form of methane monooxygenase (MMO) isolated from Methylosinus trichosporium OB3b have been studied. MMO consists of hydroxylase (MMOH), reductase, and 'B' (MMOB) components. MMOH contains a carboxylate- and oxygen-bridged binuclear iron cluster that catalyzes O2 activation and insertion chemistry. The diferrous MMOH-MMOB complex reacts with O2 to form a diferrous intermediate compound O (O) and subsequently a diferric intermediate compound P (P), presumed to be a peroxy adduct. The O decay reaction was found to be pH-independent within error at 4 °C (k(obs) = 22 ± 2 s-1 at pH 7.7; k(obs) = 26 ± 2 s-1 at pH 7.0). In contrast, the P formation rate was found to decrease sharply with increasing pH to near zero at pH 8.6; the observed rate constants fit to a single deprotonation event with a pK(a) = 7.6 and a maximal formation rate at 4 °C of k(P) = 9.1 ± 0.9 s-1 achieved near pH 6.5. The formation of P was slower than the disappearance of O, indicating that at least one other undetected intermediate (P*) must form in between. P decays spontaneously to the highly chromophoric intermediate, compound Q (Q). The decay rate of P matched the formation rate of Q, and both rates decreased sharply with increasing pH to near zero at pH 8.6; the observed rate constants fit to a single deprotonation event with a pK(a) = 7.6 and a maximal formation rate at 4 °C of k(Q) = 2.6 ± 0.1 s-1 achieved near pH 6.5. No pH dependence was observed for the decay of Q. The formation and decay rates of P and the formation rate of Q decreased linearly with mole fraction of D2O in the reaction mixture. Kinetic solvent isotope effect values of k(H)k(D) = 1.3 ± 0.1 (P formation) and k(H)/k(D) = 1.4 ± 0.1 (P decay and Q formation) were observed at 5 °C. The linearity of the proton inventory plots suggests that only a single proton is transferred in the transition state of the formation reaction for each intermediate. If these protons are transferred to the bound oxygen molecule, as formally required by the reaction stoichiometry, the data are consistent with a model in which water is formed concurrently with the formation of the reactive bis μ-oxo-binuclear Fe(IV) species, Q.