Abstract: An improved cryogenic separation process is provided for forming a substantially CO-free and lower hydrocarbon-free hydrogen-rich gas, suitable for use in producing an ammonia synthesis gas, from a hydrogen-containing gas stream also containing carbon monoxide and lower hydrocarbon impurities, wherein the hydrogen containing gas is subject to a two-stage autorefrigerated cryogenic flash treatment to remove concentrated methane-containing and carbon monoxide-containing streams, and the thus-purified hydrogen gas is passed to a nitrogen wash tower in counter-current contact with liquid nitrogen to form the desired hydrogen-rich product streams as an overheads from the wash tower.