The photocatalytic degradation of 4-nitrophenol in aqueous solution was per-formed by using a continuous fixed bed annular photoreactor. Titanium dioxide was used as the photocatalyst, supported as thin film on Pyrex glass beads. The influence of different operative parameters as substrate and oxygen concentrations, volumetric flow rate and incident photonic flow on the kinetics was studied. The Langmuir-Hinshelwood model has been satisfactorily fitted to the experimental results and its parameters have been determined. The true value of the kinetic constant can not be determined as the model contains a parameter very difficult to be measured, i.e. the total surface area active for the occurrence of photoreaction.