Abstract: A method of treating CO.sub.2 rich gas for injection into a petroleum reservoir is described. CO.sub.2 rich gas containing methane, nitrogen, ethane, propane, butanes, and heavier components is distilled such that substantially all of the heavier components are produced as a bottoms product and substantially all of the CO.sub.2, ethane, and propane are produced as an overhead vapor. The presence of ethane and propane in the overhead vapors overcomes the effect of nitrogen and methane on minimum miscibility pressure in the reservoir, and the problems created by CO.sub.2 freezing and the CO.sub.2 /ethane azeotrope are avoided, furthermore, a readily marketable liquid product is produced.