A study was made to obtain a better understanding of the manner in which preozonation affects trihalomethane (THM) formation with respect to humic acid, citric acid, and 11 aromatic compounds which were components of humic acid, all in dilute aqueous medium. Ozonation before chlorination is able tonotonly decrease, butalso increase the concentrations of THM precursors. The compounds which are inherently high THM precursors, such as resorcinol, aniline, salicyl-aldehyde, phenol, andp-hydroxybenzoic acid, are controlled with respect tothe increase inthe amount ofozone consumed, but the compounds which have essentially no or alow potential for chloroform formation, such as hydroquinone. salicylic acid, methoxybenzene. benzaldehyde. and benzole acid, produce their maximum THM levels at a certain value of ozone consumed. When 1mg of organic carbon consumes more than 8 mg of ozone, THM formation from thetested aromatic compounds isdepressed by ozone.