A method for sub-cooling normally gaseous hydrocarbon mix­tures produced in a cryogenic process unit wherein the mixture (1) is introduced to a gas/liquid separator (4), which may be a storage vessel, and vapor (8) containing at least two components of the mixture is recovered as refrigerant, employed in an open cycle refrigeration system to sub-cool the hydrocarbon mixture, and returned to the separator. The system is particularly useful for sub-cooling a hydrocarbon product stream while, at the same time, recovering boil-off vapor from a cryogenic storage vessel.