The crystal structure of the clathrate delta form of syndiotactic polystyrene containing 1,2-dichloroethane is presented. Polymer chains in s(2/1)2 helical conformation and 1,2-dichloroethane molecules are packed in the monoclinic unit cell with axes a = 17.11 Angstrom, b = 12.17 Angstrom, c = 7.70 Angstrom and gamma = 120 degrees according to the space group P2(1)/a. The crystalline density of the clathrate delta form is 1.23 g cm(-3) with two chains (eight monomer units) and two I,2-dichloroethane molecules inside the unit cell. Molecules of 1,2-dichloroethane occupy isolated cavities between the benzene rings of adjacent polymer chains and are essentially only in the trans conformation. The location and the conformation of the 1,2-dichloroethane molecule are such that both chlorine atoms are sandwiched between two benzene rings of the same polymer chain.