The photocatalytic degradation of three dihydroxybenzenes (hydroquinone, catechol and resorcinol) by TiO2 and Cu-TiO2 has been studied. Degradation rates depend on the molecule catalyst surface interaction type. FTIR studies have shown that Cu presence modifies the organics interactions with the catalysts surface. Therefore, catalysts have shown different types of behaviour with each compound. Resorcinol interaction with Cu-TiO2 may produce a hydroxyphenolate thus reducing the number of activating centres in the aromatic ring. However, catechol interaction provides a catecholate bidentate which virtually deactivates the aromatic ring, hence reducing its degradation by (OH)-O-. radicals attacks. Hydroquinone interaction with Cu-TiO2 occurs by means of a hydrogen bond, while with TiO2 this interaction is established through the formation of a hydroxyphenolate monodentate. This justifies the improved catalytic behaviour of Cu-TiO2,. Studies in presence of H2O2 have shown the existence of similar degradation mechanisms, but degradation rates have been notably higher. © 2005 Elsevier B.V. All rights reserved.