An ESR investigation is reported of oxygen radicals formed by UV radiation of silica gel surfaces in the presence of O2. The species O-2 and O-3 are identified. The O-2 radical shows a temperature dependent signal shape between -196 and 25¿°C which is interpreted in terms of Brownian rotational surface diffusion about an axis perpendicular to the O–O bond and to the surface. Correlation times for surface diffusion are estimated from comparison of observed and computer simulated spectra, and an activation energy for surface diffusion of 2.4±0.8¿kJ¿mol-1 calculated. Comparison with previously reported spectra for W/SiO2 and Ti/SiO2 surfaces indicates that the mobile O-2 species formed in all three systems must be located on the SiO2 support, and that the mobility is due to the absence of strong interaction with transition metal sites.