Effects of Fenton reagents, iron and hydrogen peroxide, on the generation of hydroxyl radicals ((OH)-O-center dot) during photo-Fenton degradation of acetaldehyde dissolved in water were examined. The dynamic change in (OH)-O-center dot radical concentration during the acetaldehyde degradation by the photo-Fenton process was measured using a coumarin fluorescence probing technique. The (OH)-O-center dot radical concentrations increased with increasing initial Fe concentrations within the Fe dosage range examined in this study. With increasing acetaldehyde loading the (OH)-O-center dot radical concentration decreased. The (OH)-O-center dot radical concentration changed rather complicatedly as the acetaldehyde degradation by the photo-Fenton oxidation process proceeded. The dynamic change in (OH)-O-center dot radical concentration could be divided into four phases: the (OH)-O-center dot radical concentration instantaneously and significantly increased just after the start of photo-Fenton oxidation process (Phase I), subsequently decreased (Phase II), increased again after reaching its minimum value (Phase III), and gradually decreased after the (OH)-O-center dot radical concentration reached a maximum point (Phase IV). This complicated change in (OH)-O-center dot radical concentration during photo-Fenton degradation of acetaldehyde could be rationally described by the Fe-redox cycle via the Fe-organic complex formation and its oxidation by dissolved oxygen besides the Fenton and photo-Fenton reactions. It was found that the oxidation of iron-organic complexes formed by ferric iron with acetaldehyde and its degradation products accelerated the rate of Fe-redox cycle. On the other hand, the formation of Fe-organic complexes led the decrease in Fe ions available for the Fenton and photo-Fenton reactions. (C) 2011 Elsevier B.V. All rights reserved.