Poly(I-trimethylsilyl-1-propyne) (PMSP) is a glassy polymer that contains many microvoids. The permselectivity of the PMSP membrane is controlled by the microvoids, and its permeation behavior is different from rubbery polymer membranes. PMSP membrane was filled with poly(1H,1H,9H-hexadecafluorononyl methacrylate) (PHDFNMA), and the solution-diffusion mechanism during pervaporation was investigated. Separation performance was increased due to the introduced hydrophobic polymer, PHDFNMA, as compared to the, PMSP membrane. For the 62 wt % PHDFNMA-filled PMSP membrane., the permselectivity was significantly enhanced. The microvoids in the PMSP membrane play an important role in permselectivity. As the feed of ethyl butanoate (EBU) concentration increased and the EBU quantity sorbed into the PHDFNMA-filled PMSP membrane increased, the diffusion of water was prevented; in turn, the water flux decreased. At a low feed concentration, the diffusivity of the EBU molecule was much lower than that of water due to the larger molecular size of EBU. As EBU was sufficiently sorbed into the membrane, the diffusion of water was prevented by the EBU molecules; in turn, the permselectivity of EBU significantly increased. At a high-feed EBU concentration, the diffusion of water increased, and the diffusion of EBU decreased to a constant as the PHDFNMA-filled PMSP membrane was plasticized,