The interaction of a coadsorbed mixture of acetone and oxygen with a clean oxidized polycrystalline sample of TiO2 (P25) was investigated using electron paramagnetic resonance (EPR) spectroscopy. UV illumination of the sample at low temperature (100 K) generated an unstable radical intermediate at the dehydrated (and hydrated) TiO2 surface. The radical decayed irreversibly at temperatures greater than 150 K but was regenerated by subsequent irradiation at low temperatures. Using a series of isotopically labeled gases (CH3COCH3, CD3-COCD3, O-16(2), and O-17(2)) to aid in the interpretation of the EPR spectrum, the radical was identified as an alkylperoxy species RCH2OO. with the spin Hamiltonian parameters of g(1) = 2.0345, g(2) = 2.0070, g(3) = 2.0010, (H)A(1) = 0.34 mT, (H)A(2) = 0.10 mT, (H)A(3) = 0.29 mT, (170)A(\\)(i) = 9.45 mT (for (ROO.)-O-17), and (170)A(\\)(ii) = 5.52 mT (for (ROO.)-O-17). By consideration of the different mechanistic pathways involved in the oxidation of acetone, it was concluded that the observed radical is generated initially by hole transfer to the adsorbed acetone and the identity of the unstable peroxy intermediate must be CH3COCH2OO..