The photolysis of cyanocobalamin by visible light in the presence of nicotinamide at pH 1.0-7.0 has been studied. The second-order rate constants for the bimolecular interaction of these vitamins have been determined which vary from 9.50 x 10(-3) (pH 1.0) to 1.25 x 10(-3) M(-1) min(-1) (pH 7.0). The rate-pH profile indicates a gradual slow decrease in rate in the pH range 1-3 followed by a relatively fast decrease in the pH range 3-7. The protonated form of cyanocobalamin appears to be more susceptible to photolysis than the non-ionised form. Cyanocobalamin photolysis leads to the formation of hydroxocobalamin at pH 1.0-7.0.