A heatless fractionating apparatus and method wherein a pressurized gaseous mixture, such as air, is alternately passed through first and second adsorbent beds for adsorbing at least a first component but not a second component of the gaseous mixture, and wherein each adsorbent bed is purged of the adsorbed component during the intervals in which the gaseous mixture is not passed through it. A valve system for each adsorbent bed comprises a pressure-responsive valve which responds to the delivery of the pressurized gaseous mixture through an electrically operated valve to deliver the gaseous mixture to its associated adsorbent bed. The pressure-responsive valve also operates to vent purge and other gases from its associated adsorbent bed when the electrically operated valve is closed to block delivery of the pressurized gaseous mixture to the pressure-responsive valve. The electrically operated valves for the two adsorbent beds are cyclically and alternately opened by a pressure-responsive control circuit, which may be equipped with one or more battery-operated alarm devices for signalling loss of electrical power and failure to switch delivery of the pressurized gaseous mixture from one bed to the other within a pre-selected time period. To quickly achieve a high concentration of the desired component in the product effluent, all of the unadsorbed gas leaving the adsorbent beds may be made available for purging the beds for a short pre-selected time period at start-up. Alternatively, stored unadsorbed gas may be made available to purge one or both of the beds at shutdown.