Transition metal loaded TiO2 powders were characterised and tested for the photodegradation of some organic acids. Both Lewis and Bronsted surface acid sites were present in the samples containing large amounts of Mo, V and W. The Bronsted sites are associated to the presence of the transition metals as they do not exist in the unloaded support, and in particular to the presence of these metals in high oxidation states. The trend of the pzc values of the various powders matches rather well both with the surface acidity of the samples experimentally determined by NH3 adsorption and with a scale of acidity/basicity for bulk binary oxides. XPS investigation indicates that only the presence of Cu(II) is able to modify the TiO2 valence band spectrum. The photodegradation results have confirmed that the presence of transition metal ions generally reduces the photocatalytic activity of TiO2. Surface acidity addresses the reactivity for the degradation of methanoic and benzoic acid: the more basic the surface of the powders the higher the rate of substrate disappearance. No relation with the acidity has been found in the case of ethanoic acid. (C) 2004 Elsevier B.V. All rights reserved.