A simple method for obtaining intrinsic kinetic parameters of photocatalytic, first-order reactions is presented. The method makes use of a microfluidic reactor, which was specifically designed to operate under conditions of negligible mass transfer limitations and involves a reversible sealing that allows one to test different catalyst films on the same device. An analytical model was derived to calculate the intrinsic kinetic constant from experimental data of reactant conversion vs flow rate. Radiation absorption by the catalyst was explicitly considered in the reaction rate expression. The degradation of a water pollutant, the pharmaceutical drug clofibric acid (CA), was evaluated in the microreactor under different flow rates, TiO2 film thicknesses, and UV irradiation fluxes. The intrinsic kinetic parameter of CA degradation was extracted from the output data of the microfluidic reactor. The model was verified against numerical calculations of the full problem (advection, diffusion and reaction). Then, as an ultimate proof of the method, the obtained value of the kinetic parameter was successfully employed to predict the performance of a conventional, laboratory scale photoreactor. The proposed method constitutes a practical tool for laboratories dedicated to modeling, design and optimization of photocatalytic reactors for environmental applications. Apart from determining intrinsic parameters, the method also enables easy and quick catalyst screening.