Abstract: For the separation of acid, carbon dioxide and/or hydrogen sulfide from a gaseous mixture comprising at least one lighter gas, e.g. methane, the gaseous mixture (1) is precooled at least once during a heat exchange operation E2; the resultant precooled gas mixture, flowing upwardly is cooled and rectified simultaneously in an approximately vertical heat exchange zone (ER) forming a downwardly-flowing countercurrent liquid reflux; the resultant rectified gaseous fraction (8) depleted in acid gases and enriched in light gas is collected at the top of the vertical exchange zone; and (d) a liquid fraction (4) enriched in acid gas components is collected at the bottom of the exchange zone. The collected liquid fraction (4) can be expanded and evaporated to provide cooling. The system has particular application for the treatment of natural gas or in petroleum refining or recovery operations.