Pore-filling-type organic/inorganic composite membranes were prepared by a plasma-graft polymerization technique. Skirasu porous glass (SPG) was used as the inorganic substrate, and methyl acrylate was used as the grafting monomer. Two preparation procedures were employed: direct plasma grafting and two-step plasma grafting. Optical microscope and micro-FT-IR studies showed that the grafted polymer was formed in the pores of the SPC; substrate. The thickness of the grafted layer formed by direct plasma grafting was about 50 mu m. With the two-step plasma grafting, the grafted layer formed was thinner. The grafted membranes showed chloroform selectivity in pervaporation and vapor permeation of a chloroform/n-hexane mixture. The separation performance of the grafted membranes was stable throughout the separation experiments. This implies that the grafted polymers are covalently bonded to the inorganic substrate. The grafted membrane prepared by the two-step plasma grafting showed higher selectivity than that of a filling-type membrane with a high-density polyethylene (HDPE) substrate. This result proved that the inorganic substrate can exhibit a high swelling-suppression effect and the filling-type membrane with an inorganic substrate can exhibit high selectivity in pervaporation separation. The membranes were thermally stable at least up to 150 degrees C.