Cytochrome c oxidase (cytochrome a03-type) [EC 1. 9. 3. 1] was purified from Erythrobacter longus to homogeneity as judged by polyacrylamide gel electrophoresis, and some of its properties were studied. The spectral properties of the oxidase closely resembled those of mitochondrial and other bacterial cytochromes aa₃. The enzyme showed absorption peaks at 430 and 598nm in the oxidized form, and at 444 and 603 nm in the reduced form. The CO compound of the reduced enzyme showed peaks at 432 and 600nm. The enzyme oxidized eukaryotic ferrocytochromes c more rapidly than E. longus ferrocytochrome c. The reactions catalyzed by the enzyme were 50% inhibited by 0.7μM KCN. The enzyme contained 1g atom of copper and 1g atom of magnesium per mol of heme a. The enzyme molecule seemed to be composed of two identical subunits, each with a molecular weight of 43, 000.