The crystal structure of the clathrate form of syndiotactic poly(p-methylstyrene) containing benzyl chloride is presented. This structure presents a monoclinic unit cell (cell constants a = 26.2 A, b = 11.8 A, c = 7.9 A and gamma = 116 degrees) in which the s(2/1)2 polymer helices and guest molecules are packed according the space group P2(1)/a. This molecular complex represents, to our knowledge, the first case of polymeric clathrate form in which in the same crystal are present two kinds of cavities having different shape and dimensions: alpha class cavities (containing two guest molecules) and beta class cavities (containing one guest molecule). The contemporaneous presence of these two types of cavities is originated by a disorder in the arrangement of right- and left-handed helices in the crystals related to the particular staggered layout of the layers of chains containing the cavities in which helices of a given chirality can be randomly substituted by enantiomorphous ones.