An interlaboratory study in which 20 laboratories participated was conducted to provide precision and accuracy statements for the proposed EPA Method 604-Phenols for measuring concentrations of the Category 8 chemicals phenol, 2,4-dimethylphenol, 2-chlorophenol, 4-chloro-3-methylphenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, pentachlorophenol, 2-nitrophenol, 4-nitrophenol, 4,6-dinitro-2-methylphenol, and 2,4-dinitrophenol in municipal and industrial aqueous discharges. The method provides for the determination of the phenols by gas chromatography (GC) with flame ionization detection (FID) or derivatization and detection by electron capture (EC). The study design was based on Youden's plan for collaborative tests of analytical methods. Three Youden pair samples of the test compounds were spiked into six types of test waters and then analyzed.