We will study the soluble methane monooxygenase (sMMO) from Methylococcus capsulatus (Bath), which activates dioxygen for incorporation into a variety of substrates. Our previous XAS studies have afforded a structural characterization of the oxidized, mixed-valent, and reduced states of the sMMO hydroxylase component, and the interaction among the hydroxylase, coupling protein, and substrate. We propose (i) to utilize XAS for interrogating intermediate species formed in the reaction of reduced hydroxylase and coupling protein with dioxygen; (ii) to study species formed in the reaction of hydroxylase with nitric oxide (NO), a surrogate for the initial reaction of O2, in the presence and absence of the coupling protein; (iii) to study model complexes designed to illuminate our understanding of these types of dinuclear systems.