Formation of .OH radicals during TiO2 photocatalyzed oxidations driven by simultaneous UV and microwave radiation was probed by the electron spin resonance technique employing a novel setup in which the ESR sample (contained the DMPO spin-trap agent and P-25 TiO2 or pure anatase or pure rutile in aqueous media) could be irradiated by both UV light and microwave radiation; the microwave generator was coupled through an N connector to the ESR sample cavity and to a photoreactor in which phenol was photooxidized by simultaneous irradiation with UV light and microwaves to assess whether the photooxidative process correlated with the concentration of OH radicals formed. (C) 2003 Elsevier B.V. All rights reserved.