Deuterium isotope separation process takes place in the reaction of the amalgamated aluminium metal surface and water. The reaction produces hydrogen gas with ~40 ppm deuterium content and the formed aluminium hydroxide gel is enriched in deuterium. The theoretical separation factor is s=3.9. Besides the kinetic isotope effect, the key factor in the separation process is the H2O+HD=HDO +H2 exchange reaction. Since only a low amount of amalgam forms on the aluminium surface, the overvoltage difference of the deuterium and the hydrogen on the mercury does not play key role in the process. The main role of the amalgam formation is embodied in the decomposition of the stable oxide layer on the aluminium surface, thus ensuring free metallic surfaces for the reaction with the water.