The one-component transient permeation of carbon dioxide through an alumina-supported silica membrane is studied for pressures up to 2.0 × 10⁷ Pa, at temperatures of 296, 313, and 358 K. The permeation is obtained for gaseous, liquid, and supercritical carbon dioxide. For all conditions high carbon dioxide fluxes were obtained. The membrane has been tested with a maximum pressure difference between the feed and the permeate side of about 2.5 × 10⁶ Pa. At a feed-side pressure of 2.0 × 10⁷ Pa and at 358 K the permeance is equal to 8.0 × 10^-8 mol m^-2 s^-1 Pa^-1. The transient transport through the microporous membrane can be described with a single mass-transfer coefficient. For a feed-side pressure between 1.0 × 10⁵ and 200 × 10⁵ Pa and a temperature of 296 K the mass-transfer coefficient increases by a factor of 80. The fact that carbon dioxide has a high flux through this type of silica membrane opens the way for regeneration of carbon dioxide at supercritical conditions.