Cyanide binds to fully reduced cytochrome bo and induces a blue shift of the Soret absorption band of the high-spin heme o and a change in the visible region spectrum consistent with the expected conversion to a low-spin state. The dissociation constant, determined by titration of the extent of the binding spectrum, is 7.0 +/- 0.6 mM at pH 7.0. In contrast, cyanide does not bind significantly in this concentration range to the reduced form of cytochrome bd. The reduced cyanide compound of cytochrome bo can be laser photolyzed. Typically, less than 20% photolysis was attained with conditions that give essentially full photolysis of the carbon monoxide compound. The subsequent monophasic kinetics of recombination of cyanide at varying cyanide concentrations were used to determine kon, koff, and dissociation constant values at pH 7.0 of 572 +/- 43 M-1 s-1, 4.2 +/- 0.7 s-1, and 7.3 +/- 1.3 mM, respectively. The dissociation constant changes very little in the pH range 6-8, indicating that a proton is bound together with the cyanide anion, as predicted by our recent proposal of a requirement for electroneutrality in the binuclear center [Mitchell, R., & Rich, P. R. (1994) Biochim. Biophys. Acta 1186, 19-26]. Competition studies confirm that cyanide and carbon monoxide cannot bind simultaneously, so that their binding sites must overlap. A small fraction of the reduced unliganded enzyme appears to have a distinct photolysis spectrum in the absence of added ligands, and this is transformed into a typical ferrous cyanide compound only at very high cyanide concentrations. Cyanide binding and photolysis were also examined in a number of mutant forms of cytochrome bo, and in a wild-type form which was partially depleted in CuB. Dramatic changes in rate constants and binding constants were found in several cases. Data from several mutants were compared with analogous data on the binding and photolysis of carbon monoxide, and the effects of mutation were quite different with the two ligands. A model is developed to explain the observed effects of point mutations on the recombination kinetics of both carbon monoxide and cyanide. Overall, the results indicate that the CuB site is required for binding of cyanide, but not carbon monoxide, to the reduced enzyme, possibly by providing the site for binding of the associated proton.