A process to separate hydrocarbon gases into different fractions by condensing in the high pressure condenser of a staged dual pressure rectification system only that amount of vapor necessary for the reboiler duty of the low pressure column and expanding the excess vapor to provide the low temperature refrigeration duty. The process of the invention lends itself particularly to refinery gases, particularly to an absorber tail gas which comprises a mixture of low molecular weight hydrocarbon gases (e.g. methane, ethane, ethylene, propane, etc.).This disclosure also describes a process for the low pressure fractional distillation of gaseous mixtures with relative volatilities of less than 2 by compressing part of the overhead gas product, removing the heat of compression with cooling water, cooling it by heat exchange with the overhead vapor stream from the low pressure column, condensing it in the high pressure condenser to supply additional reboiler duty to the low pressure column and adding the liquid to the reflux stream to the low pressure column.