Kinetic studies of the reactions of selected eukaryotic and prokaryotic cytochromes c with mitochondrial cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase (EC 1.9.3.1) using a standardized complex IV preparation from beef heart are reported. Data on reactions with NADH-linked cytochrome c reductase (complexes I and III) are included. The concentration ranges employed provide a basis for quantitative demonstration of a general rate law applicable to oxidase reactions of cytochrome c of greatly differing reactivities. Results are interpreted on the basis of a modified Minnaert mechanism (Minnaert, K. (1961) Biochim. Biophys. Acta 50, 23), assuming productive complex formation between cytochrome c and free oxidase in addition to further complex binding of a second cytochrome c molecule to the initially formed oxidase complex. Kinetic constants so obtained are consistent with the assumption that binding is the dominant parameter in reactivity, and can be rationalized most simply on this basis.