Solid-state structure of syndiotactic polystyrene (s-PS) after crystallization from the melt and the glassy state was examined by differential scanning calorimetry, density and X-ray diffraction analysis (WAXS). It was possible to prepare semicrystalline s-PS containing pure alpha (alpha" or alpha' modifications) and pure beta crystalline forms with different crystallinities (0-40%). The measurements confirmed the low density of both crystalline forms, which in the case of alpha crystalline form was smaller and in the case of beta crystalline form was slightly larger than the density of the glassy amorphous s-PS. Oxygen and carbon dioxide gas permeability, diffusion and solubility of semicrystalline s-PS containing different crystalline forms were studied as a function of crystallinity. These measurements confirmed that more dense beta crystalline form was impermeable for the transport of small gas molecules while less dense alpha crystalline form was highly gas permeable. Unusual gas transport behavior of the alpha crystalline form was attributed to porous crystalline structure containing the nanochannels. Despite the porous structure, alpha crystalline form showed very low oxygen and carbon dioxide solubility compared to gas solubility in the amorphous phase. The proposed diffusion model explained the characteristic features of the gas permeation behavior for chemically 'inert' small gas molecules in the permeation medium consisting of glassy amorphous polymer with dispersed porous crystalline phase containing the nanochannels. A new relaxation process with the maximum at about -35 degreesC was observed in semicrystalline s-PS containing the a crystals and was associated with the porous structure of this crystalline form.