Commercial pentachlorophenol (PCP) contains significant quantities of tetrachlorophenol (TCP). The occurrence of TCP in environmental samples provides a chemical marker for PCP originating from commercial formulations. Negative chemical ionization mass spectrometry has been used to examine a commercial PCP formulation and a series of environmental and human samples. Tetrachlorophenol was determined by the ion current at m/z 229, tetrachlorophenoxide, and PCP was determined by the ion current at m/z 267, pentachlorophenoxide. The ion current at m/z 267 may include contributions from the oxygen/chloride exchange product of hexachlorobenzene, an environmental precursor of PCP. The ratio of PCP to TCP in Dowcide G-ST, a commercial PCP formulation, was 2.5 plus or minus 0.1. The ratio of m/z 267 to m/z 229 in a jellyfish, Mnemiopsis macrydi, from the Gulf of Mexico was 2.7 plus or minus 0.1, in human semen it was 4.1 plus or minus 0.1, and in human adipose tissue it was 15.5 plus or minus 0.1. PCP in the semen samples was concentrated in the sperm cells by a factor of 9.