The cytochrome oxidases are a wide and heterogeneous family of electron transfer hemoproteins which play a central role in the energy metabolism of aerobic organisms. They catalyze the terminal electron transfer steps in the complex branched respiratory systems of prokaryotes as well as in the mitochondrial respiratory chain [1]. The structural and functional variety of these enzymes is perhaps exemplified by P. denitrificans where, depending on the growth conditions, up to five different terminal oxidases termed aa 3 , cytochrome o, cytochrome a 1 , cytochrome d and cytochrome cd 1 can be synthesized [2]. The differences among the various oxidases are consistent. Copper and heme a, the redox centers present in the mitochondrial enzyme, may be replaced in the bacterial terminal oxidases by different prosthetic groups, while cytochrome c and oxygen may not always be the electron donor and acceptor. Also the integration in the membrane does not appear as a general feature. Depending on their spectroscopic properties and prosthetic groups, the cytochrome oxidases have been arranged in different classes [2, 3]. Nevertheless, the structure and the kinetic competence of most of them remain poorly understood. In this situation, changes of class and nomenclature are not surprising [4, 5]. Only recently the structural organization of these proteins in bacteria began to be elucidated at molecular level. In E. coli the two terminal oxidases, termed cytochrome d and cytochrome o, have been characterized.