A novel single-crystal membrane [Cu-(II)(2)(4-F-bza)(4)(2-mpyz)](n) (4-F-bza = 4-fluorobenzoate; 2-mpyz = 2-methylpyrazine) was synthesized and its identical permeability in any crystal direction in the correction for tortuosity proved that gas diffuses inside the channels without detour. H-2 permeated by 1.18 X 10(-12) mol m m(-2) s(-1) Pa-1 with a high selectivity (F alpha: 23.5 for H-2/CO and 48.0 for H-2/CH4) through its 2D-channels having a minimum diameter of 2.6 angstrom, which is narrower than the Lennard-Jones diameter of H-2 (2.827 angstrom), CO (3.690 angstrom), and CH4 (3.758 angstrom). The high rate of permeation was well explained by a modified Knudsen diffusion model based on the space expansion effect, which agrees with the observed permselectivity enhanced for smaller gases in considering the expansion of a channel resulting from the collision of gas molecules or atoms onto the channel wall. An analysis of single-crystal X-ray data showed the expansion order to be H-2 > Ar > CH4, which was expected from the permeation analysis. The permselectivity of a porous solid depends on the elasticity of the pores as well as on the diameter of the vacant channel and the size of the target gas.