The luminescence of Cu-substituted [horse heart] cytochrome c exhibits temperature-dependent decay modes and spectral shifts at temperatures below 80.degree. K. By comparison with the data from metal-substituted porphyrins, intramolecular rate parameters were determined. The observed tripdoublet decay time is 13 .+-. 1 .mu.s at 77.degree. K and the quartet decay time is 12 .+-. 5 .mu.s below 30.degree. K. The luminescence emission from Cu cytochrome c in the presence of cytochrome c depleted mitochondria and in the presence of soluble cytochrome c oxidase was also investigated. Upon binding, the intensity and decay time of the emission are altered. The results indicate an interaction betwen cytochrome c and its mitochondrial binding site which depends on the electronic state of cytochrome c.