Two FIA methods for the determination of pentachlorophenol, based on its oxidizing-condensation reaction with 4-aminoantipyrine in aqueous medium and spectrophotometric detection in the conventional and stopped-flow modes, are described. In both cases the calibration graphs are linear in the range from 1.0 to 60.0 mug/ml pentachlorophenol, with sampling rates of 48 and 28 samples/hr for triplicate analysis, respectively. Interferences from phenol and other chlorophenols have been evaluated. The stopped-flow method is particularly useful for determining pentachlorophenol in commercial formulations containing no more than 15% of other chlorophenols.