This work describes the first ever direct detection of HO2 radicals in the gas phase above photocatalytic surfaces. A glass plate covered with TiO2 has been illuminated in the presence of H2O2 by a 20W fluorescence lamp centred at 365 nm. The activity of the photocatalytic material has been proven through direct, time resolved observation of the degradation of H2O2 by following its concentration by the very sensitive and selective technique of cw-Cavity Ring Down Spectroscopy (cw-CRDS). An absorption line of H2O2 at 6639.89 cm(-1) has been used, permitting a detection limit of [H2O2]min = 1.3 and 3.6 x 10(13) cm(-3) for 50 and 200 Torr of synthetic air, respectively. A lower limit of the quantum yield for H2O2 degradation has been estimated to phi(min) = 0.0024. Under the same conditions, the formation of HO2 radicals has been detected directly and selectively in the gas phase, using the same technique. HO2 radicals have been observed at up to 4 cm above the surface and at total pressures of up to 230 Torr. (C) 2010 Elsevier B.V. All rights reserved.