NOVELTY - Process for generating an essentially carbon monoxide-free hydrogen/helium gas stream involves feeding a high-purity hydrogen/helium feed stream for greater than or equal to 30 minutes through a first adsorber of two adsorbers, and recovering a first essentially carbon monoxide-free hydrogen/helium gas stream from first adsorber; and purging first adsorber by depressurizing it to less than atmospheric pressure, and by recycling a portion of the carbon monoxide-free hydrogen/helium gas stream through first adsorber, and/or feeding an inert gas stream through first adsorber. The adsorber has a zeolite adsorbent.    USE - For producing carbon monoxide-free hydrogen or helium gas stream, and feeding the carbon monoxide-free hydrogen or helium gas streams to a hydrogen or helium distribution system which includes a hydrogen fuel station for vehicles (claimed); and also useful for producing nitrogen, and methane-free hydrogen and helium from high-purity (e.g. pipeline grade) hydrogen or helium gas streams; useful in various applications including hydrogen-powered fuel cell or internal combustion vehicles, stationary power applications, backup power units, power grid management, power for remote locations, and portable power applications in consumer electronics, business machinery.    ADVANTAGE - The adsorbents used in process can be regenerated rapidly by evacuation and purging (e.g. within around 10-30 minutes) since removed impurities are relatively weakly adsorbed. A recycle stream of essentially carbon monoxide-free hydrogen is used to purge the system by simple evacuation at sub-atmospheric pressure. Since the amount of purge gas required is very small compared to the volume of gas treated, very high hydrogen recoveries (e.g. greater than around 99.5%) are obtained. The process does not require the equipment and energy needed for thermal regeneration and can operate over extended periods of time (e.g. around four hours). By using physical adsorbents with high heats of nitrogen adsorption, intermediate heats of carbon monoxide adsorption, and low heats of hydrogen and helium adsorption, and by using vacuum purging and high feed stream pressures (e.g. pressures of as high as around 1000 bar), pipeline grade hydrogen or helium can purified to produce essentially carbon monoxide-free hydrogen and helium, or carbon monoxide, nitrogen, and methane-free hydrogen and helium. The vacuum swing adsorption process allows the hydrogen recovery of 99.6%.    DETAILED DESCRIPTION - Process for generating an essentially carbon monoxide-free hydrogen or helium gas stream, involves feeding a high-purity hydrogen or helium feed stream for greater than or equal to 30 minutes through one or a first of two adsorbers that are fluidly connected in parallel or in series; and recovering a first essentially carbon monoxide-free hydrogen or helium gas stream from the first adsorber; and purging the one or first adsorber by depressurizing it to less than atmospheric pressure, and by recycling a portion of the first essentially carbon monoxide-free hydrogen or helium gas stream through one or first adsorber, and/or feeding an inert gas stream through the one or first adsorber. The adsorber is packed with an adsorbent consisting of a zeolite which is cation exchanged with a metal other than a transition metal and which contains a transition metal (less than 0.5 wt.%). The zeolite is in binderless form. The adsorbent has a heat of hydrogen or helium adsorption of less than 5 kcal/mole, a heat of carbon monoxide adsorption of 8-12 kcal/mole, and a heat of nitrogen adsorption of greater than 5 kcal/mole. The high-purity hydrogen or helium gas feed streams are fed to the first and second adsorbers at a pressure of 30-1000 bar. The adsorbers are packed with adsorbent particles having a particle size of 0.5-5 mm.