A zeolite membrane was formed on a porous alpha-alumina support tube by hydrothermal reaction using a solution which was appropriate for the synthesis of ETS-4 zeolite. The morphology and x-ray diffraction pattern of the top surface of the support tube were consistent with those of the ETS-4 zeolite. The permeances to single-component H-2, CO2, O-2, N-2, and CH4 were in the range (0.8-2.5) x 10(-8) Mol m(-2) sec(-1) Pa-1, while those to n-C4H10 and iso-C4H10 were less than 10(-11) mol m(-2) sec(-1) Pa-1. This suggests that the membrane functioned as a molecular sieve with pores smaller than 0.4 nm. At a permeation temperature of 3 1 OK, the ideal N-2/O-2 separation factor was 2.8, and the N-2/O-2 separation factor was 2.3 and 3.2 for a mixture of N-2/O-2 = 1 and 4, respectively. Thus, the membrane is nitrogen-selective and can be used to enrich the oxygen level in air, which is fed on the pressurized side, without much loss of compression energy.