The photocatalytic oxidation of 4-nitrophenol in the presence of solid TiO2 has been studied in an annular reactor. The experiments have been performed in both acidic and basic solutions (pH = 3 and 8.5) at different catalyst loads and temperatures. The concentration-time data have been correlated with the rate equation: -d[R]/dt = k(0)k(2)[R]/(1 + k(2)[R]) in order to evaluate the unknown parameters, The laws of dependence of the parameters k(0) and k(2) on catalyst load and temperature have been interpreted in the terms of detailed kinetic models proposed in the literature. The results throw some light on the different reaction mechanisms occurring on varying the operating conditions.