The degradation of high phenol and 1,2-propylene glycol concentrations (1 g/l) in water solutions by TiO2-photocatalysis has been studied. Important differences between the degradation mechanism of both molecules have been observed. From the obtained data it may be suggested that degradation of phenol takes place onto the catalyst surface by means of formation of peroxo-compounds, At low phenol concentrations another mechanism, the insertion of OH. radicals may be favored. At low or high 1,2-propilene glycol concentrations degradation seems not to be affected by the OH. radical insertion process. These conclusions have been obtained by studying the effect of catalyst concentration and adding different OH. radical precursors such as H2O2 and S2O82-. Degradation intermediates have been identified by HPLC and FTIR. The FTIR study of the catalyst surface has shown infrared bands attributable to different chemisorbed M-OC6H5 species, peroxo-compounds, formates, ortho-formates and carboxilates that can inactivate the catalyst. (C) 2001 Elsevier Science B.V. All rights reserved.