Photocatalytic degradation of 1,3 -dihydroxy-5-methoxybenzene (1,3-DHMB) is investigated in the presence of TiO2. Effects of initial concentration of 1,3-DHMB, pH of the medium, irradiation time, and temperature are examined. Degradation of 1,3-DHMB obeys zero-order kinetics. Following the formation of CO2, the reaction rate constant, k, is found as 217 mu mol m(-3) s(-1) and the adsorption constant, K, is found as 4.22 m(3) mol(-1). The highest degradation of 1,3-DHMB and the highest concentration of CO2 are both obtained at pH 9.0. The apparent activation energy is calculated as 17.1 kJ/mol. For the route of degradation, a mechanism is postulated.