This work was made to study the photochemistry of riboflavin and the kinetics of riboflavin degradation after exposure to UV-Vis light phosphate buffer solution at different pH interval (2-10) with the same concentration (5×10-5M ) was studied by the of UV-Vis spectrophotoscopy analyses, it showed that degradation increase as pH increase from pH (2-7)where the lumichrome a photoderivative forms in the acidic medium. a maximum degradation occurs at neutral pH (7). Then it began to decrease from pH (7-10). Where the lumiflavine (a photoderivative occurs at basic media) formed. We also calculated the quantum yield of this decomposition and found that an increasing occur from pH (2-7) (1.902×10-4-6.020×10-4) then a decrease occur from pH (7-10) (6.020×10-4- 1.266×10-4) as well as the reactivity ratio.