Poly(stearyl methacrylate-co-methacrylic acid) (SMA) and its sodium ionomer (SMI) were synthesized and the permeability of the model drug through the SMA and SMI films was measured. The side-chain crystalline structure for the dried and hydrated SMA, SMI was investigated using DSC and WAXS. The side-chain crystalline structure of the hydrated SMI was much more stable than that of the hydrated SMA at room temperature. The temperature-sensitive phase transition of the side-chain crystalline structure for the hydrated SMI was also studied by the temperature variable WAXS experiment. The temperature-sensitive permeation of the hydrophilic model drug through SMI was observed around 20 degrees C, whereas the drug permeation through SMA was almost constant within the temperature range studied, The change of drug permeability through the SMA and SMI films with temperature seems to be associated with the side-chain crystalline structure of the polymer.