Abstract: A system for purifying gas in which one or more main adsorbers containing a gas-adsorbent comprising a molecular sieve carbon (MSC) or the like is connectable in series to a supplementary adsorber containing a substantially smaller amount of the gas-adsorbent in the range of from 5 to 50% of the adsorbent contained in the main gas-adsorbers. The main gas-absorbers are connectable in parallel so that when the gas-adsorbent in one of the main absorbers is being regenerated, and vice versa, the other main gas-adsorber can function independently to adsorb gas or can be connected in series with said supplementary gas-adsorber so that the purification of gas can be continuously effected while one or the other of the main gas adsorbers is being regenerated.