In the course of a study of the mechanisms of the reaction of chlorine dioxide (CI'O,) with phenols', we sought a method whereby dilute aqueous reaction mixtures, sometimes containing excess oxidant, could be separated and rhe products analyzed without prior quenching ad extraction_ Mixtures of chlorophenols and chlorinated benzoquinones and hydroquinones resulting from treatment of dihydric phenols with aqueous hypochlorite have been analyzed by gas chromatography (GC) without derivatization', but the procedure required quenching and extraction and was not suitable for large numbers of product studies of fast reactions in very dilute solutions. However, in recent years hi&performance liquid chromatography (HPLC)=, with the use of both normaI and reversedJs5 phases, has been the method of choice for determination of phenols and chiorophenols in dilute aqueous solutions. We here describe a reversed-phase HPLC method by which standard samples of the chlorophenols. chlorobenzoquinones and chlorohydroquinones anticipated as possible products of the reaction of CiOz with phenol and with hydroquinone were separated and quantified, and by which unquenched o.xidation mixtures were analyzed directly-EXPERIMENTAL Chenllcals All solutions were made with deionized doubly distilled water. Acetonitrile was Mallinkrodt chromatographic grade. Phenoi (MCB reagent) was purified by distil-Iation under nitrogen; p-benzoquinone (Baker reagent) and 2chlorohydroquinone (Pfaia and Bauer) were steam distilled. Hydroquinone (AIdrich), 4-chlorophenol (Chemical Service Co.) and 2.6-dichlorobenzoquinone (Pfaltz and Bauer) were purified by recrystallization. 2-Chlorophenol (Chemical Service Co.) and 2,4-dichlorophenol (Eastman) were used as received. 2,6-Dichlorobenzoquinone was prepared from 2,4,6-trichloropheno16, and 2,3_dichiorobenzoquinone from 2,3_dichlorophe-noI' by CrO,-acetic acid o_xidat:on. 2,6-and 2,3-Dichlorohydroquinones were prepared by treatment of the respective dichlorobenzoquinones with NaBH, in ethanol.