The effect of caffeine complexation with riboflavin on the kinetics of riboflavin photolysis in the pH range 2.0-10.5 has been studied. The photolysis of riboflavin solutions (5x10(-5) M) was carried out in the presence of caffeine (0.5-2.5x10(-4) M) using a visible radiation source. A specific multicomponent spectrophotometric method was used for the determination of riboflavin and photoproducts in photolysed solutions. The apparent first-order rate constants (k) for the photolysis reactions range from 2.71x10(-4) to 4.26x10(-2) min(-1). The values of the rate constants decrease with increasing concentrations of caffeine indicating its inhibitory effect on the reactions. The second-order rate constants (k') for the caffeine inhibited reactions lie in the range of 0.13 to 5.10x10(-3) M(-1) min(-1). The log k-pH profiles for the photolysis reactions at various caffeine concentrations involve multiple steps indicating a gradual increase in the rate up to pH 10. The lower rates at pH 2.0 and 10.5 are due to the ionization of riboflavin as evident from fluorescence measurements. The k'-pH profile for the interaction of riboflavin with caffeine represents a bell-shaped curve in the pH range 3-6 followed by a sigmoid curve in the pH range 7-10. The inhibition of photolysis of riboflavin in the presence of caffeine is a result of the monomeric interaction and complex formation of caffeine with riboflavin. The photochemical interaction of riboflavin with caffeine suggests that a pH around 6 is most appropriate for the stabilization of the vitamin. At this pH the complex shows the highest stability constant.