Compound C2 is a product of the reaction of O2 and the mixed-valence state of cytochrome oxidase. The mixed-valence state of membrane-bound cytochrome oxidase is obtained at -24 degrees C, by using either ferricyanide or yeast peroxidase complex ES as oxidants, and the configurations of oxidized haem a and its associated copper (a3+Cua2+) and of reduced haem a3 and its associated copper (ac3+.CO.Cua3+) are obtained. The mixed-valence-state cytochrome oxidase mixed with O2 at -24 degrees C and flash-photolysed at -60 to -100 degrees C reacts with O2 and initially forms an oxy compound (A2) similar to that formed from the fully reduced state (A1). Thereafter the course of the reaction differs from that obtained in the fully reduced state, and absorbance increases are observed at 740--750 nm and 609 nm and a decrease at 444 nm, with no increase in absorbance at 655 nm. One possible attribution of the absorbance increases is to charge-transfer interaction between the iron of haem a3 and the copper associated with haem a3, Cua3(2+), having properties of a type-I 'blue' copper. A possible attribution of the decrease in absorbance at 444 nm is to liganding of a3(2+). A related explanation is that the 609 nm absorbance involves a charge-transfer interaction of both iron and copper as a mixed-valence binuclear complex, Cua3, having properties of a non-blue copper. Intermediates in addition to Compound C2 are not yet identifiable by chemical or spectroscopic tests. The kinetic and equilibrium properties of Compound C2 are described.