It has been demonstrated that the modification of the TiO2 acid-base properties by the presence of activated carbon (AC) considerably modify the interaction of some molecules with the catalyst surface and hence its photocatalytic characteristics.FTIR studies have shown that while phenol interacts with TiO2 yielding a phenoxide, in the catalyst low AC containing catalysts mixtures the surfacial hydroxylic groups are inserted in the aromatic ring in the symmetric position to the -OH group. Additionally, the 4-aminophenol study shows that the molecule interacts with the TiO2 surface by means of the amino group, while in the catalyst containing AC the interaction takes place through the hydroxylic group. These results have confirmed the catalyst acid-base properties changes determined by the characterisation studies already performed. Also, it has been observed that catalysts with the lowest AC content show better catalytic behaviour than the untreated TiO2 and those with higher AC load. Another interesting result is the efficiency of these catalysts under solar irradiation. (C) 2003 Elsevier Science B.V. All rights reserved.