Dicyclopentadiene, a waste product of manufacturing at the Rocky Mountain Arsenal near Denver Colorado, has been detected in ground water at this facility. Ground water extracts were analysed by gas chromatography-mass spectrometry (GC/MS) to determine if derivatives of dicyclopentadiene were present in addition to dicyclopentadiene and other chemical wastes. The derivatives thus identified were characterized by GC high resolution MS, deuterium exchange for active hydrogen in the chemical ionization source of the mass spectrometer, and GC/MS following on-column base catalyzed deuteration of enolizable hydrogen. Two ketone derivatives of dicyclopentadiene were identified by comparison with synthetic standards. Ground water derivatives of dicyclopentadiene were compared with mammalian enzymatic metabolites produced in vitro by incubation with immobilized rabbit liver cytochromes P-450. The metabolites were analysed by GC/MS and by GC/MS following derivatization and deuterium labelling. Although not found in ground water samples, the two metabolites were identified as monoepoxides of dicyclopentadiene by comparison with compounds synthesized in the laboratory.