Aq. polyoxyethylene(n=10) [and, for comparison, (8) and (4)] isooctylphenyl ethers (Triton X-100 [X-114, X-45]), were treated with 120 mg O-3/L for 0-60 min. In aq. 20-mmol/L X- 45, 114 or 100 solution and with 20 mg O-3/L at inlet, the initial reaction rates were 0.025, 0.055 and 0.058 moll(L(.)s) as evaluated from COD data; the higher the n, the more rapid the decomposition. The X-100 decomposed in 40 min, presumably in two steps (rupture of benzene ring followed by destruction of polyoxyethylene chain). At micelle concentrations less than critical (CMC), X-100 decomposed faster, 31.9% in 30 min. At CMC, O-3 could hardly migrate to X-100 molecules and the X-100 loss was 1.5% only. Toxicity toward bacteria Vibrio fischeri NRRL B-11177 rose; presumably on account of formaldehyde formed. As the decomposition progressed, the BODICOD ratio increased, indicating the rise in biodegradability.