Humic acids dissolved in water are decomposed by ozone under u.v. irradiation at pH 6.9. The decomposition rate of the total organic carbon (TOC) is obtained as functions of the u.v. intensity and the residual ozone concentration, assuming the reaction obeys first order kinetics with respect to reactive TOC concentration. Major final products are acetic acid, formic acid and oxalic acid, which are refractory to ozonation and are no longer precursors of trihalomethanes. The amount of chloroform and total organic chlorides generated by chlorination are drastically decreased by u.v. irradiation. No increase in the trihalomethane formation potential is observed in the course of u.v. ozonation.
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