Abstract: A purification system for producing extremely pure H.sub.2 from H.sub.2 gas streams containing water and small amounts of O.sub.2 comprises a single column with alternating layers of adsorbent and a catalyst for the reaction 2H.sub.2 + O.sub.2 .fwdarw. 2H.sub.2 O. The alternating layers are arranged so that the gas to be purified first meets an adsorbent layer, next a catalyst layer and finally an adsorbent layer. Two such columns can be operated alternatively, one being used to produce purified gas, while the other is backflushed with as little as 2% of the product gas of the operating column. By this technique, H.sub.2 gas can be produced having a purity as high as 99.9999% or higher. The inventive system can also be used to purify O.sub.2 gas streams containing water and small amounts of hydrogen.