The events which follow photolysis of CO-inhibited fully reduced and CO-bound mixed-valence cytochrome o have been studied in two variants of the enzyme, one of which contains heme B at the low-spin site (bo3) and the other of which contains heme O (oo3). For this, isolated enzyme was prepared from three different strains of Escherichia coli which produce these two variants in different relative amounts [Puustinen, A., Morgan, J. E., Verkhovsky, M., Thomas, J. W., Gennis, R. B., & Wikström, M. (1992) Biochemistry 31, 10363-10369]. In both types of enzyme microsecond electron redistribution was observed from the oxygen-binding heme to the low-spin heme. In the bo3 enzyme, the rate was similar to that in the bovine enzyme (3 microseconds), but in the oo3 enzyme, it was several times slower. However, in both types of cytochrome o, the same electron redistribution process was also apparently observed on other time scales, some faster and some slower. The rate of CO rebinding in the mixed-valence enzyme was found to be slower than in the fully reduced enzyme, apparently because of the subpopulation of oxidized oxygen-binding heme produced by the electron redistribution. The extent of this electron redistribution, and thus the inter-heme delta Em, can be calculated from this change in rate. The heme B and heme O containing low-spin sites have Em values about 20 and 50 mV lower, respectively, than the oxygen-binding heme.