The physicochemical properties of TiO2 particles in the diameter range between 6.6 and 30.1 nm and the effect of the crystalline size on arsenic adsorption and photocatalytical oxidation were investigated. TiO2 nanoparticles of different sizes were single-phase anatase. The adsorption capacity of the TiO2 for As(III) and As(V) increased linearly with the N-2 Brunauer-Emmett-Teller surface area (S-BET) of the particles. There was not much difference in the rate of As(III) photooxidation when the diameter of the TiO2 nanoparticles was between 6.6 and 14.8 nm. However, the As(III) photooxidation rate clearly decreased when the particle size increased to 30.1 nm. Arsenite photooxidation data could be fitted with a first-order kinetics equation. Published by Elsevier B.V.