The heterogeneous photocatalytic oxidation of nitrite to nitrate over TiO 2 in aqueous medium has been investigated. The amount of nitrite photo-oxidized was determined at different irradiation times, initial nitrite concentrations, pH, mass of semiconductor in suspension and incident light intensities. It has been observed that the yield of the nitrite photo-oxidation increases sharply with decreasing pH, as does the amount of nitrite adsorbed on TiO 2 particles. This suggests that the process must take place at the adsorption layer. From the experimental results it has been deduced that the photoreaction follows a Langmuir-Hinshelwood kinetics, with a rate constant of 2.3 × 10 -4 mol l -1 min -1 and an equilibrium adsorption constant of 0.29 10 3 l mol -1 . © 1991.