Upon irradiation by a light flash (100-J), the carbon monoxide complex of cytochrome P-45Oscc was fully photodissociated in both the presence and absence of cholesterol, while less than 20% of the CO complex was photodissociable with those of deoxycorticosteronebound and -free forms of cytochrome P-450118. When the quantum yield of the reaction was measured for each photodissociable portion, the values were 0.5 and 1.0 for the substrate-free and -bound forms of cytochrome P-45Oscc, and 0.03 and 0.8 for the substratefree and -bound forms of cytochrome P-450118, respectively. Thus, CO complexes of these enzymes become more photosensitive upon binding with the specific substrates. Steroid binding also affected kinetic constants of reactions between the ferrous enzymes and CO. The rate constants for the CO recombination at 15 'C were 2.7 X lo6 and 2.3 X 10' "ls-l for the substrate-free and -bound forms of cytochrome P-45Oscc, and were 7.0 X lo6 and 5.4 X 10' "'s-l for the substrate-free and -bound forms of cytochrome P-450118, respectively. The rate constants for the CO dissociation also decreased upon the steroid bindings. The products of the enzyme reactions, pregnenolone and corticosterone, had similar effects on the kinetic constants. From these findings, we postulate that the binding of a steroid to the substrate site of each enzyme alters the bonding character of CO with the heme-iron, thereby affecting both photochemical and kinetic properties of the CO complex. The nature of the photoindissociable portion of the CO complex of cytochrome P4501 18 is also discussed.