The reaction of fully reduced soluble bovine heart cytochrome oxidase with O2 at 173K was investigated by low-temperature optical and e.p.r. spectroscopy, and the kinetics of the reaction were analysed by non-linear optimization techniques. The only e.p.r. signals seen during the course of the reaction are those attributable to low-spin cytochrome a3+ and CuA2+. Quantitative analysis of e.p.r. signals shows that, at the end point of the reaction at 173K, nearly 100% of CuA is in the cupric state but only about 40% of cytochrome a is in the ferric low-spin state. The optical spectra recorded at this stage of the reaction show incomplete oxidation of haem and the absence of a 655 nm absorption band. The only reaction scheme that accounts for both the e.p.r. and optical data is a four-intermediate mechanism involving a branching pathway. The reaction is initiated when fully reduced cytochrome oxidase reacts with O2 to form intermediate I. This is then converted into either intermediate IIA or intermediate IIB. Of these, intermediate IIB is a stable end product at 173 K, but intermediate IIA is converted into intermediate III, which is the stable state at 173 K in this branch of the mechanism. The kinetic analysis of the e.p.r. data allows the unambiguous assignments of the valence states of cytochrome a and CuA in the intermediates. Intermediate I contains cytochrome a2+ and CuA+, intermediate IIA contains low-spin cytochroma a3+ and CuA+, intermediate IIB contains cytochrome a2+ and CuA2+, and intermediate III contains low-spin cytochrome a3+ and CuA2+. The electronic state of the O2-binding CuBa3 couple during the reoxidation of cytochrome oxidase is discussed in terms of an integrated structure containing CuB, cytochrome a3 and O2.