The electron spin resonance (ESR) spin-trapping technique using 5,5-dimethyl-1-pyrroline-N-oxide as the spin-trap reagent has been applied to detect free radical intermediates generated during in situ ultraviolet or visible irradiation of aqueous 4-chlorphenol (4-CP)/N-doped TiO2 suspensions. ESR measurements gave the first direct evidence that the active species ((OH)-O-center dot and O-2(center dot-)) are responsible for the photodecomposition of 4-CP over N-doped TiO2 under visible-light irradiation, strongly suggesting that the photocatalytic reaction of organic compounds in powdered N-doped TiO2 Systems proceed via surface intermediates of oxygen reduction or water oxidation, not via direct reaction with holes trapped at the N-induced midgap level. These results have important implications for the evaluation of the oxidative powder of TiO2-N-x(x) catalysts.