The influence of lattice distortions on TiO 2 photocatalysis produced by subjecting commercially available P25 titania samples to a heat treatment in the temperature range 645–800°C was examined; it caused the initial anatase-to-rutile ratio of 81/19 to decrease to 1/99 at the highest temperature. The photoactivities of these heat-treated samples were established through the photodegradation of 4-chlorophenol (4-CP) exposed to UV irradiation alone, UV/microwave irradiation (2.45GHz), and to UV irradiation accompanied with conventional heating at a temperature otherwise identical to that under UV/MW irradiation. Raman band intensities of pure anatase (143cm −1 ), pure rutile (446cm −1 ) and of the heat-treated P25 specimens were examined in situ after being exposed to microwave irradiation for about 4min. Changes are attributed to a microwave non-thermal effect involving oxygen vacancies that affect the specimens’ photoactivities as determined by subjecting samples of Evonik P25 titania (AEROXIDE ® TiO 2 P25) and Ishihara ST-01 TiO 2 to a heat treatment in the presence of molecular hydrogen. Such treatment caused lattice distortions of both systems that affected the kinetics of degradation of the chlorophenol under various irradiation conditions. UV/visible absorption spectra of the heat/H 2 -treated specimens displayed a broad unresolved absorption envelope at wavelengths above 400nm that has been attributed to oxygen vacancies and thus to F-type color centers in accord with an earlier study by Kuznetsov and Serpone. The presence of such defects, particularly in the heat/H 2 -treated samples, and the influence of the microwaves caused the photodegradation kinetics for the 4-CP to be enhanced significantly.