We have determined the crystal structures of bicyclo[2.2. 1]hept-2-ene-endo-cis-5,6-dicarboxylic anhydride, C(9)H(8)O(3), (I), 1,2,3,4,7,7-hexachlorobicyclo[2.2.1]hept-2-ene-endo-cis-5, 6-dicarboxylic anhydride diethyl ether solvate, C(9)H(2)Cl(6)O(3).0. 16C(4)H(10)O, (II), bicyclo[2.2.1]hept-2-ene-endo-cis-5, 6-dicarboxylic acid, C(9)H(10)O(4), (III), 1,2,3,4,7, 7-hexachlorobicyclo[2.2.1]hept-2-ene-endo-cis-5,6-dicarboxylic acid, C(9)H(4)Cl(6)O(4), (IVa) and (IVb), and ethyl 1,2,3,4,7, 7-hexachloro-6-carboxybicyclo[2.2. 1]hept-2-ene-endo-cis-5-carboxylate monohydrate, C(11)H(8)Cl(6)O(4). H(2)O, (V). Compounds (I) and (II) were prepared by a standard Diels-Alder reaction from maleic anhydride and cyclopentadiene or hexachlorocyclopentadiene, respectively. The crystal-growing processes of these compounds led to surprising results: rapid recrystallization of (I) from diethyl ether and (II) from petroleum ether gave crystals of these compounds, however, crystallization by slow evaporation techniques using common solvents yielded new compounds in which the five-membered heterocycle has been cleaved.