The reaction between a cytochrome oxidase from Pseudomonas aeruginosa and oxygen has been studied by a rapid mixing technique. The data indicate that the heme d1 moiety of the ascorbate-reduced enzyme is oxidized faster than the heme c component. The oxidation of heme d1 is accurately second order with respect to oxygen and has a rate constant of 5.7 - 10(4) M-1 - s-1 at 20 degrees C. The oxidation of the heme c has a first order rate constant of about 8 s-1 at infinite concentration of O2. The results indicate that the rate-limiting step is the internal transfer of electrons from heme c to heme d1. These more rapid reactions are followed by more complicated but smaller abcorbance changes whose origin is still not clear. The reaction of ascorbate-reduced oxidase with CO has also been studied and is second order with a rate constant of 1.8 - 10(4) M-1 - s-1. The initial reaction with CO is followed by a slower reaction of significantly less magnitude. The equilibrium constant for the reaction with CO, calculated as a dissociation constant from titrimetric experiments with dithionite-reduced oxidase, is about 2.3 - 10(-6) M. From these data a rate constant of 0.041 s-1 can be calculated for the dissociation of CO from the enzyme.