The photocatalytic degradation of phenol, guaiacol, 2-chlorophenol and catechol in aqueous suspensions of TiO2 under different experimental conditions has been investigated. The photodegradation of the different organics follows a Langmuir-Hinshelwood kinetics, showing rate constants that decrease in the order: guaiacol > 2-chlorophenol congruent to phenol > catechol. A similar trend is also observed, except for catechol, for the stability of the a-complexes that may be formed between the aromatic ring and the OH. radical. From different analytical techniques (HPLC, GC/MS and HPLC/MS), various hydroxylated intermediate compounds have been reported acid different mechanisms of degradation of the starting compounds have been proposed. From experiments performed using aqueous solutions containing the four organics, it is observed that the competitive Langmuir-Hinshelwood kinetics for the degradation of each one of the phenolic compounds is obeyed. (C) 2001 Elsevier Science B.V. All rights reserved.