The photocatalytic degradation of catechol and resorcinol with TiO2 in presence of acetic acid has been studied. Low concentrations of the acid (30-50 ppm) can modify the adsorption and degradation of the dihydroxybenzenes. However, the acetic acid concentration threshold at which effects are observed depends on the compound. FTIR analyses have shown that due to steric factors, catechol and resorcinol molecules adsorb on different TiO2 surface centres. Additionally, acetic acid molecules are able to displace Ti4+ coordinated water molecules and those interacting by hydrogen bonds, to a lesser extent. Consequently, the catalyst surface becomes restructured, favouring the adsorption of the dihydroxybenzenes. The acetates reaction with holes and oxygen molecules with photogenerated electrons to give O-2(center dot-) radicals which react with OH groups with acidic character to give new radicals that enhance catechol and resorcinol degradations. (c) 2005 Elsevier B.V. All rights reserved.