Aqueous solution of polyethylene glycol (PEG) was treated by ozone under UV irradiation (15 W lowpressure mercury lamp) at pH 2, 7 and 11. The degradation of PEG by ozone was enhanced by UV irradiation under all pH conditions. Under acidic conditions under which PEG was not degraded without UV irradiation, the UV light enhanced remarkably the consumption of ozone and the lowering of molecular weight but the TOC of the solution did not appreciably declined. The production of aldehydes by

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... ion was enhanced also by UV irradiation under all pH conditions. Formaldehyde which is inhibitory to bactreria, however, was very small under neutral. UV irradiation enhanced exceedingly also the TOC decline under neutral conditions.