To confirm that long flexible polymeric compounds are capable of forming cocrystals with syndiotactic polystyrene (sPS) by occupying more than one cavity in the crystalline lattice, the complexation of sPS with poly(ethylene glycol) dimethyl ethers of molecular weight 2000 (PD2000) was carried out through an additive-assisted guest-exchange procedure. The X-ray fiber and powder diffraction and IR spectrum of sPS films treated by the procedure showed that the original guest chloroform was completely substituted with PD2000, keeping the basic structure of the host sPS lattice. It has been clarified by gravimetric analysis and IR spectroscopy that the molar ratio of styrene monomer to PD2000 is 16, which can be justified only by assuming the multiple site occupation of PD2000 in the sPS complex. It is strongly suggested that longer flexible polymeric molecules can be introduced into sPS lattices in a similar manner.