Parallel experiments under similar conditions, using various substrates (atrazine, fenitrothion, 4-chlorophenol and 2,4-D) and OH radical scavengers (Br-, isopropyl alcohol, tertiary butyl alcohol and acetone), have shown that the photooxidizing mode of PW12O403- and TiO2, i.e., OH radicals and/or holes (h(+)), depends on the nature of substrate and the mode of investigation. This provides an explanation for the controversial results reported in the literature. Atrazine shows that both PW12O403- and TiO2 operate, mainly, via OH radicals and to a lesser extent with holes (h(+)), whereas, fenitrothion suggests that both systems operate almost exclusively, via OH radicals. Differences in the action of the catalysts are encountered in the photodegradation of 4-chlorophenol (4-ClPh) and 2,4-dichlorophenoxyacetic acid (2,4-D). PW12O403- appears to operate essentially via OH radicals, whereas, h(+) appear to be the major oxidant with TiO2. Overall, though, the action of OH radicals relative to h(+) appears to be more pronounced with PW12O403- than TiO2. (c) 2006 Elsevier B.V. All rights reserved.