The composition of technical chlordane has been investigated by combined gas chromatography/mass spectrometry, employing a column commonly used for pesticide residue analysis. Partial or complete structure identification have been assigned to some 45 individual constituents. Many of these compounds are simple Diels-Alder adducts of cyclopentadiene and hexachlorocyclopentadiene, with further chlorine addition or substitution. Additionally, a series of adducts derived from tetra- and pentachlorocyclopentadiene, followed by further chlorination, has been identified. Several components appear to have incorporated trichloromethyl moieties from the solvent during manufacture. Other compounds with abnormal or rearranged ring fusions have also been recognized and their unusual mass spectra have been studied.