Since the micropore size of Y-type zeolite is in the range of 0.74-1.2 nm, small molecules are able to pass one another inside the micropores. Thus ion-exchanged Y-type zeolite membranes are expected to show catalytic activity as well as being useful for gas separation with high permeation rates. In this study, NaY-type zeolite membranes were synthesized by a hydrothermal reaction on the inner surface of a porous alpha -alumina support tube. The Y-type zeolite membranes were stable to heat at temperatures up to 400 degreesC, and the permeances to single-component hydrocarbons showed a maximum at temperatures that were dependent on the heat of adsorption. The permeances to C3H6, n-C4H10 and i-C4H10 decreased with time at elevated temperatures. This can be explained by the deposition of carbonaceous matter, which was removed by a heat treatment in an atmosphere of O-2 at 300-400 degreesC. The permeation mechanism for the NaY-type zeolite membrane was investigated for single-component CO2. The Maxwell-Stefan diffusivities were compared with the Fickian diffusivities obtained using a sorption-diffusion model, in which linear concentration profiles across the membrane were assumed. The differences between the Fickian diffusivities and the Maxwell-Stefan diffusivities were negligible at elevated temperatures.