The crystal structures of two molecular complex phases of syndiotactic polystyrene (s-PS) with 1,3,5-trimethyl-benzene (TMB) and 1,4-dimethyl-naphthalene (DMN) have been described. These structures present a monoclinic unit cell in which the s(2/1)2 polymer helices and guest molecules are packed according the space group P2(1)/a and unit cell constants: a = 17.3 angstrom, b = 15.4 angstrom, c = 7.8 angstrom and gamma = 95.7 degrees for s-PS/TMB and a = 17.4 angstrom, b = 17.2 angstrom, c = 7.8 angstrom and gamma = 116.4 degrees for s-PS/DMN. Both structures can be described as intercalates, since they present ac layers of polymer helices alternated to layers of contiguous guest molecules and a guest/monomeric-unit molar ratio of 1/2, as recently observed only for s-PS/norbornadiene molecular complex. On the basis of a comparison between crystalline structures and X-ray diffraction data of several s-PS molecular complexes, a simple criterion to anticipate their clathrate or intercalate nature has been suggested. (c) 2006 Elsevier Ltd. All rights reserved.