Aq. polyoxyethylene (av. 9.5) isooctylphenyl ether (Triton X-100) solns., 30, 120 or 200 mg/l., were photooxidized 5 hrs./20 degrees C with 10 l. O(2)/hr. at UV 4, 8, 15, or 150 W. Decompn. of X-100 as followed by spectrophotometry and COD, proceeded on photolysis and on photooxidn., 20% in O(2)-free (Ar-satd.) and 75% in O(2)-bubbled solns. In 5-hr.-long photooxidn., at 8 and 150 W, ChOD fell by 12 and 35%, resp. Photooxidn. decompd. the X-100 mols., rather than shortened the oxyethylenic chain. Peroxide radicals formed and the benzene ring was ruptured.