It has been clarified that syndiotactic polystyrene (sPS) forms co-crystalline structures with polyethylene glycol dimethyl ethers (PEGDMEs) with molecular weights ranging from 178 to 1 000 g . mol(-1) through a guest exchange procedure assisted by a plasticizing agent. The PEGDME molecules are incorporated into the spaces between sPS polymer sheets consisting of (T(2)G(2))(2) helices. The results of X-ray diffraction and gravimetric measurements suggest that one or less molecules are included per unit cell for PEGDME with average molecular weight of 1 000 g . mol(-1), which indicates the possibility that longer polymeric molecules can be introduced into sPS lattices with multiple site occupation.