Heterogeneous photocatalysis in a water solution is recognised as a method of totally eliminating most recalcitrant organic pollutants found in such solutions. Our current work has tried to show that the heterogeneous photocatalysis process can also be a selective elimination method in the case of the mixture: 4-hydroxybenzoic acid (4-HBz) and benzamide (Bz) at semi-pilot plant scale (190 dm(3)) under conditions of solar irradiation. The photocatalysis experiments performed by us were done at the "Plataforma Solar de Almeria" and were carried out in the compound parabolic collectors' CPC system. We investigated in particular the influence of TiO2 loading, the effect of 4-HBz concentration and the effect of the presence of chloride anions and pH on the selectivity of the reaction process. (C) 2004 Elsevier Ltd. All rights reserved.