Volcanic ash, surface-water, and bottom-material samples obtained in the vicinity of Mount St. Helens after the May 18, 1980, eruption were analyzed for organic contaminants by using capillary gas chromatography-mass spectrometry-computer techniques. Classes of compounds identified include n-alkanes, fatty acids, dicarboxylic acids, aromatic acids and aldehydes, phenols, resin acids, terpenes, and insect juvenile hormones. The most probable source of these compounds is from pyrolysis of plant and soil organic matter during and after the eruption. The toxicity of selected compounds and their environmental significance are discussed.