|Abstract:||Sewage effluent was analyzed for 3,5,6-trichloropyridinol (TCP) by extracting one liter of water using liquid-liquid extraction and determined by GC/MS operated in the negative ion chemical ionization (electron capture) mode, TCP is the major metabolite of the commonly used insecticide, chlorpyrifos, and herbicide trichlorpyridinol. The recoveries using dichloromethane as the extractant were 59% with a relative standard deviation of 2% This method was used to investigate levels of TCP in sewage effluent. During this analysis a tentatively identified additional isomer of TCP (X-TCP) was found. The 3,5,6-TCP, the common chlorpyrifos metabolite and the synthesized isomer, 3,4,5-TCP were compared with X-TCP. All three isomers have significantly different retention times. The average level of 3,5,6-TCP was 3.4 ng/L while the level of X-TCP was 39.8 ng/L.