tA study of the photodegradation of 5 × 10−5M riboflavin (RF) in 0.2–1.0 M phosphate buffer in the pres-ence and absence of 2.50 × 10−4M caffeine at pH 6.0–8.0 has been carried out. RF in phosphate buffer isphotodegraded simultaneously by normal photolysis (photoreduction) and photoaddition reactions giv-ing rise to lumichrome (LC) and cyclodehydroriboflavin (CDRF) as the main final products, respectively.RF and its photoproducts, formylmethylflavin (FMF), lumiflavin (LF), LC and CDRF in degraded solutionhave been determined by a specific multicomponent spectrophotometric method with an accuracy of±5%. The apparent first-order rate constants for the photodegradation of RF and for the formation of LCand CDRF are 5.47–15.05 × 10−3min−1, 1.06–8.30 × 10−3min−1and 4.31–8.05 × 10−3min−1, respectively.An increase in phosphate concentration leads to an increase in the rate of formation of CDRF and altersthe photodegradation of RF in favor of the photoaddition reaction. This photoaddition reaction is furtherenhanced in the presence of caffeine which results in a further decrease of the fluorescence of RF in phos-phate buffer. Caffeine may facilitate the photoaddition reaction by suppression of the photoreductionpathway of RF.