The irreversible thermal denaturation of cytochrome cd1 oxidase from P. aeruginosa as a function of the oxidation-reduction states of its hemes was observed with a differential scanning calorimeter. Upon full reduction of the 4 hemes, the apparent denaturation temperature decreases by .apprx. 10.degree. C and the denaturation enthalpy decreases slightly: oxidized, 5.9 cal/g; reduced, 5.4 cal/g. At pH 7.5, the first order rate constants for denaturation at 90.degree. C are: reduced, 33 .times. 10-3 s-1; oxidized, 3 .times. 10-3 s-1. Oxidation of the hemes results in heat stabilization of the cytochrome oxidase. The activation energy for denaturation of fully reduced oxidase, 53 kcal/mol, is less than that for fully oxidized protein (73 kcal/mol).