When a gas‐filled porous medium is permitted to imbibe liquid from a source at a pressure equal to the initial gas pressure, some of the gas is trapped in the pores and is discontinuous with the atmosphere surrounding the medium. The amount of gas initially trapped varies over a range at least as wide as from about 5 to about 50% of the pore volume. The percentage of trapped gas is gradually reduced to zero as the gas diffuses away under the influence of capillary compression. This happens even though the imbibed liquid is presaturated with the gas initially trapped. Porous media become completely saturated with liquid first at exposed surfaces, and the zone of complete saturation moves continuously toward the interior.