Mononuclear chromium(I) alkyne complex (i-Pr2Ph)(2)nacnacCr(eta(2)-C-2(SiMe3)(2)) (1) reacts rapidly with dioxygen to yield chromium(V) dioxo species (i-Pr2Ph)(2)nacnacCr(O)(2) (2). The mechanism of this oxygen cleavage has been studied experimentally and computationally. Isotope labeling studies rule out a direct four-electron oxidative addition of O-2 to one chromium atom, which involves a spin-forbidden transformation. Instead, the reaction likely proceeds via an unsymmetric binuclear chromium bis(mu-oxo) complex. The latter has been independently prepared and structurally characterized. Its reactivity with O-2 is consistent with the proposed mechanism.