Abstract An analytical method for phenols hab been adapted for the analysis of chlorophenylmercapturic acids in rat urine. Chlorothiophenols were produced from the mercapturic acids by hydrolytic cleavage with sodium hydroxide. Acetate esters of the chlorothiophenols were formed by addition of acetic anhydride to the aqueous alkaline solution. After acylation, the acetate derivatives were extracted into hexane. Forming the acetate esters of the chlorothiophenols prevented their oxidation to disulfides and significantly improved their chromatographic properties. Electron-capture gas chromatographic analysis of the acetate esters was performed on a mixed phase column, 4% SE-30 + 6% OV-210. Recoveries of four chlorothiophenols ranged from 82 to 93%. This method required no sample transfer steps; therefore, sample loss and analysis time were minimized.