Zirconia membranes were prepared on porous a-alumina tubes using a sol-gel technique, and platinum was then impregnated in the pores of the membranes by dipping in 0.1 mol L-1 H2PtCl6 solution, followed by calcination in air. This procedure was repeated four times maximum. The membranes were then exposed in hydrogen prior to permeation tests. Permeation rates through the membranes were determined for single-component gases (H-2, CO2 and i-C4H10) at temperatures of 308-573 K. The permeation rates of the unmodified zirconia membrane obeyed the Knudsen diffusion mechanism. However, hydrogen permeation changed from the Knudsen diffusion to surface diffusion after the third Pt modification, and to the activated diffusion after the fourth Pt modification. The ideal H-2/CO2 and H-2/i-C4H10 selectivities for the membrane, which was Pt-modified four times with calcination at 573 K, were approximately 30 and 1000, respectively, at a permeation temperature of 523 K. When a mixture of H-2 (80 mol%) and CO2 (20 mol%) was fed, the membrane showed an H-2/CO2 selectivity higher than 400 and an 11, permeance of 4.21 x 10(-8) mol m(-2)s(-1)Pa(-1) at 523 K.