A method for the determination of pentachlorophenol, 2,3,4,6-tetrachlorophenol, and their salts or hydrolyzable biological conjugates in urine, water, serum, or fish tissue is reported. The method utilizes fused silica capillary gas chromatography (GC) with electron capture detection of the free phenols following hydrolysis and extraction into toluene. A recovery standard (4-chloro-2-nitrophenol) and an internal standard for GC (1-nitro-2,3,5,6-tetrachlorobenzene) are used to monitor recovery and GC performance. The detection limit for TCP and PCP is 0.5 ppb and the precision observed for each sample in the 1- to 100-ppb range is between 3 and 12% relative standard deviation for both components.