A sensitive and reliable method suitable for routine determination of pentachlorophenol (PCP) in natural and waste waters is reported. PCP is extracted from the preserved sample with benzene and from the benzene into a potassium carbonate solution. The addition of acetic anhydride to the aqueous solution produces the acetate derivative of PCP, which is extracted in hexane and analyzed by electron capture gas-liquid chromatography. The analysis of PCP in lake water and sewage effluent is presented, along with the retention times of 17 other phenolic acetates. By this procedure, as little as 0.01 ppb PCP in 1 L water can be determined.