A new crystalline clathrate class of syndiotactic poly(p-methylstyrene) has been found through thermal and WAXS analyses. In these clathrate forms, the polymer chains assume a T(6)G(2)T(2)G(2) conformation with a repetition period of 11.7 +/- 0.1 angstrom and t2 symmetry. The new conformation, supported by molecular mechanics calculations, presents a cavity, favorable to the formation of clathrate structure, only on one side of the polymer chain. This cavity is bigger than those presented by this polymer with a helical s(2/1)2 chain conformation found for all the other clathrates. After annealing, the clathrates of this new class are transformed in form II. For this new class of clathrates the term gamma is proposed.