The kinetics of photolysis of aqueous riboflavin solutions on UV and visible irradiation has been studied in the pH range 1-12 using a specific multicomponent spectrophotometric method for the simultaneous determination of riboflavin and its major photoproducts (formylmethylflavin, lumichrome and lumiflavin). The apparent first-order rate constants for the photodegradation reactions in the pH range have been determined. The log k-pH profiles indicate that riboflavin has maximum photostability around pH 5-6, at which the rate of oxidation-reduction of the molecule is lowest. The cationic and anionic forms of riboflavin are non-fluorescent and less susceptible to photolysis than the non-ionised molecule as indicated by the relatively slow rates below pH 3.0 and above pH 10.0. The rate of photolysis is increased up to 80-fold at pH 10.0, compared to that at pH 5.0, due to increase in redox potentials with an increase in pH and consequently the ease with which the molecule is oxidised. The increase in rate at pH 3.0, compared to that at pH 5.0, appears to be due to the involvement of the excited singlet state as well as the triplet state in riboflavin degradation. The apparent first-order rate constants for the photolysis of riboflavin at pH 5.0-10.0 with UV and visible radiation are 0.185 x 10(-2) to 13.182 x 10(-2)min(-1) and 0.098 x 10(-2) to 7.762 x 10(-2)min(-1), respectively.