A comparison was made of chlorodibenzo-p-dioxin contamination in a technical and an analytical grade of commercially available pentachlorophenols. Hexa-, hepta-, and octachlorodibenzo-p-dioxin were found in both samples. The technical product was more contaminated than the analytical one by factors of 1400, 600, and 539 for the hexa-, hepta-, and octachlorodibenzo-p-dioxin, respectively. Tetrachlorodibenzo-p-dioxin was not found. Results were determined by gas chromatography and confirmed by infrared spectrophotometry and mass spectrometry. Mass spectral analysis indicated the presence of polychlorodibenzofurans and polychlorodiphenyl ethers. A distinction was made, using gas chromatography-mass spectrometry, between the chlorodibenzofurans observed as original constituents of the pentachlorophenol and those derived from the fragmentation of chlorodiphenyl ethers.