Studies were conducted on the response of 2,4,6-trichlorophenol (1), 2,3,4,5-tetrachloro-phenol (2) and 4,5-dichloroguaiacol (3) toward advanced oxidation processes, such as UV-, O-2/UV-, H2O2/UV-, O-3/UV-and O-3-H2O2/UV-photolyses with irradiation of 254-nm photons. The compounds 1-3 are among the chlorophenols found in the Kraft-pulp bleach plant E-I effluents. The studies were extended to treatment of these compounds with ozonation and O-3-H2O2 oxidation systems in alkaline aqueous solution. Except for the O-2/UV-photolysis of 1 and H(2)0O(2)/UV-photolysis of 2, the dechlorination of 1-3 by O-2/UV- and H2O2/UV-potolyses were less effective than the corresponding N-2/UV-potolysis Of 1-3. Guaiacol-type chlorophenols were more readily able to undergo dechlorination than non-guaiacol type chlorophenols by N-2/UV-, O-2/UV- and H2O2/UV-potolyses. In addition, the efficiency for the dechlorination of 1-3 by N2/UV-, O-2/UV- and H2O2/UV-potolyses appeared to be dependent upon the inductive and resonance effects of substituents as well as number and position of chlorine substituent in the aromatic ring of the compounds. The dechlorination of 2 by treatment with O-3 alone is slightly more effective than the corresponding the O-3/UV-photlysis, whereas the dechlorination of 2 by treatment with the combination of O-3 and H2O2 was slightly less effective than the corresponding O-3-H2O2/UV-photolysis. In contrast, the dechlorination of 3 on treatment with O-3 alone was slightly less effective than the corresponding the O-3/UV-photolysis, whereas the dechlorination of 3 on treatment with the combination of O-3 and H2O2 was slightly more effective than the corresponding the O-3-H2O2/UV-photolysis. In the dechlorination of 2 and 3, chemical species derived from ozone and hydrogen peroxide in alkaline solution were dominant reactions in the O-3/UV- and O-3-H2O2/UV-photolysis systems as in the O-3 and O-3-H2O2 oxidation systems. Possible dechlorination mechanisms involved were discussed on the basis of kinetic data. (C) 2004 Elsevier Ltd. All rights reserved.