The effect of the casting solvent on the structure of poly[1-(trimethylsilyl)-1-propyne] (PTMSP) membranes was investigated experimentally. The PTMSP membranes were cast from solutions of cyclohexane, toluene, and tetrahydrofuran; the membranes were characterized by the positron annihilation lifetime spectroscopy (PALS) technique and by gas-permeation measurements of O-2, N-2, and CO2. The decay curves from the positron annihilation lifetime spectroscopy gave the best fit when two long-life components (tau(3) and tau(4), tau(3) < tau(4)) were employed. This suggests that two types of free volume existed in the PTMSP membranes. The size and number density of tau(4), which was characteristic for PTMSP, decreased in the following order of the casting solvents: cyclohexane > toluene > tetrahydrofuran. The order was consistent with the order of gas permeability. A good correlation was observed between the permeability and the structural parameter that denoted the free-volume size and the number density of tau(4).