Stable trichloro-octadecyl silane (ODS) derivatives of a 5 nm gamma-alumina ceramic membrane were prepared. Gas permeabilities of the untreated membrane did not show Knudsen diffusion at 20 degrees C. Gas permeabilities of the ODS membrane were three orders of magnitude lower; He, Ne, Ar, CO2, C3H8 have near constant permeabilities 360 mol s(-1) m(-2) bar(-1), except methane which has the highest permeability of the group, 481 mol s(-1) m(-2) bar(-1). The mechanism of diffusion is solution/diffusion. Remarkably, permeabilities of ODS-alumina membrane were reduced by 5 x after exposure to a pressure difference of 1 atm (active layer side) against vacuum for only 10 min. The effect was metastable but could be reversed on standing for several hours, reversal of pressure difference or after washing with (hydrocarbon solvent) toluene. The mechanism was presumed to be due to movement of the octadecyl-hydrocarbon chains of the silane monolayer causing a partially blocked pore structure; perhaps a unique example of self-fouling.