This manual describes analytical methods used by the U.S. Geological Survey to determine organic substances in water, water-suspended-sediment mixtures, and bottom material. Some of the analytical procedures yield determinations for specific com-pounds, whereas others provide a measure of the quantity of groups of compounds present in the sample. Examples of the first category are procedures for the organochlorine and organophosphate insecticides, chlorophenoxy acid and triazine herbicides, and specific substituted phenols. Examples of the second category are the various organic carbon analyses and the polychlorinated biphenyl methods. The analytical methods are presented in a standard format; topics covered include conditions for application of the method, a summary of the method, interferences, required apparatus and reagents, analytical procedures, calculations, reporting of results, and estimation of precision.