The photocatalytic activity of TiO2 nanoparticles of different size has been studied by following the photodegradation of dichloroacetic acid (DCA), and was tested by measuring the initial rate of the H+ and Cl- ion formation. The catalyst efficiency was found to be in correlation with the specific surface area of the semiconductor particles. The efficiency of the nanocrystalline TiO2 photocatalyst could be increased by doping the particles with copper ions. The structure and the partitioning of different surface copper complexes has been studied using ESR technique. The influence of the pH value of the solutions, as well as of the adsorbed copper ion content, on the efficiency of the DCA photodegradation is discussed in the paper.