The paramagnetic susceptibilities of the transition metal components (cytochromes [alpha] and [alpha]3 and copper) of cytochrome oxidase, oxidized, reduced and in the presence of cyanide and carbon monoxide, have been determined. Assuming that reduced cytochrome a3 is the only component responsible for the volume magnetic susceptibility changes of the reduced enzyme preparation upon the addition of cyanide or carbon monoxide, the results obtained with a sensitive volume magnetic susceptometer and electron spin resonance spectrometers are analyzed. Cytochrome a3 in the oxidized form has a molar susceptibility of 7, 900 X 10-6 cgs emu, which is intermediate between known data of oxidized hemoproteins of the low and high spin types, whereas cytochrome a is a hemoprotein of the low spin type magnetically similar to cytochrome c. Although copper is found to be responsible for part of the susceptibility changes in some part of the results, no evidence in favor of a role of copper in the oxidase mechanism is found.