The subsituted niobocene compound [Cp’2NbCl](x) (1, Cp’ = eta(5)-C(5)H(4)SiMe(3)) is an electron-rich Nb(III) system that reacts readily with oxygen and other potential oxygen atom donors. The ultimate product of these processes is the known Nb(V) oxo compound Cp’Nb-2(=O)(Cl) (2), which obviously arises by way of a multistep mechanism. We have investigated the mechanism of the autoxidation using a series of alternate syntheses and/or modified reaction conditions and provide direct evidence for a sequence involving the sequential formation of the eta(2)-peroxide Cp’Nb-2(O-2)Cl, its reaction with excess 1 to generate 2, the subsequent conproportionation of 2 and 1 to make the mu-oxo compound [Cp’2NbCl](2)(O) (3), and the further oxidation of this compound to 2. The reaction of 1 and 2 is postulated to involve the intermediacy of a peroxide-bridged system [Cp’2NbCl](2)(O-2) (5); although this compound has not been observed directly, attempted synthesis of 5 using superoxide lead directly to 3. These individual reactions are kinetically competent, and the results thus constitute a four-step mechanism for the production of 3. In addition, studies on the reactions of 3 with PMePh(2) illustrate that 3 disproportionates to 2 and 1 and that the latter is trapped rapidly. Conversely, reaction of 3 with additional dioxygen does not require dissociation and may proceed via a direct redox pathway. Finally, the oxygen atom transfer process involved in dioxygen reduction is compared to reactions involving nitrosoarenes and carbon dioxide, processes that yield azoxyarene (or azoarene) and carbon monoxide, respectively; mechanistic features differ, even though the overall process involves X=O scission in all cases.