SiO2/TiO2/phosphate was obtained by the sol-gel processing method, having the following characteristics: specific surface area S-BET = 800 m(2) g(-1), Ti = 14.8 wt% and P = 1.5 wt%, and ion exchange capacity of 0.58 mmol g(-1). The tetrakis(1-methyl-4-pyridyl) porphyrin ion, H2TmPyP4+, was immobilized on the matrix surface by an ion exchange reaction and then metallated in situ with Co(II), resulting in SiO2/TiO2/phosphate/CoTmPyP material. The amount of CoTmPyP incorporated to the matrix was 35.0 mu mol g(-1). Cyclic voltammetry studies and rotating disk electrode experiments using a carbon paste electrode made with the material were carried out. The immobilized complex catalyzed O-2 reduction to H2O at -0.22 V in 1 mol L-1 KCI solution at pH 6.8. The cathodic current intensities plotted against O-2 concentrations, between 1 and 11 ppm, showed a linear correlation. (C) 2000 Academic Press.2301195199