A reactivateable adsorption filter assembly is described in which to reactivate the adsorption filter element (11) a purge valve (15) is opened causing filtered fluid stored in the storage vessel (18) to flow back through the adsorption filter element (11) and out of the filter assembly via the purge valve (15). A flow restrictor (19) is positioned between the storage vessel (18) and the filter element (11) to provide a large volume of fluid containing adsorbate at a very low partial pressure during the reactivation process.