The prosthetic groups of cytochrome o, the terminal ubiquinol:dioxygen oxido-reductase of Escherichia coli, were investigated in the purified and in situ enzyme. The interactions between the redox-active centres, of which there are three, were characterised using optical, magnetic resonance and X-ray absorption spectroscopy techniques. The copper complement of this enzyme was investigated and the available data suggested a single copper centre associated with the ligand binding haem centre (haem o) forming a binuclear oxygen binding and reduction site. Two redox-active copper atoms are known to exist in the mammalian cytochrome c oxidase complex and the consequences of the different copper stoicheiometries, in these enzymes, are discussed. Spatial and organisational investigations are described and a model for the binuclear reaction site presented. The location of the haem centres was determined using low-temperature EPR spectroscopy by observing the effects on the relaxation behaviour of these centres in the presence of an extrinsic paramagnetic probe.