NOVELTY - A gaseous mixture (1) is pre-cooled in a heat exchanger (E2). It is then cooled and rectified simultaneously in a vertical heat exchange zone (ER), forming a descending liquid reflux. A gaseous fraction depleted in acid gas and enriched in lighter gas is collected at the head of the vertical exchange zone. A liquid fraction enriched in acid gas is collected in the base of the exchange zone.    USE - To separate carbon dioxide and hydrogen sulfide from a mixture containing lighter gases.    ADVANTAGE - The method aims at economy and a minimum number of plant items. Energy consumption is reduced. The process uses simultaneous refrigeration and rectification of the gaseous mixture in an indirect heat exchanger. In an example, solvent inhibits solid carbon dioxide crystal formation and allows a lower temperature e.g. -50 degrees C to be reached in the head, reducing the quantity of acid entrained in the lighter gas.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for corresponding plant.    Preferred features: The liquid fraction collected, is at least partially let down and vaporized, satisfying at least some of the refrigeration demand during rectification. Gaseous fractions generated by let down, are separated from the liquid rich in acid gases and recycled to the first stage. In a variant, let down is staged through at least two pressure levels. The liquid from the first stage of let-down, is contacted in counter current flow, with a vapor reflux arising from the liquid fraction collected below the contact zone. Further variations based on the foregoing principles are provided. Rectification takes place at a pressure of 10-70 bar. At least part of the liquid fraction enriched in acid gas, is pumped for re-injection underground, using a designated well. An external refrigeration source is employed for fractionation.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic flow diagram.    Gaseous mixture (1)    Heat exchanger (E2)    Vertical heat exchange zone (ER)