Experimental studies of adsorption and desorption of molecular hydrogen on an amorphous porous solid water ice surface between 10 and 35 K reveal a very efficient isotopic segregation process. A statistical model, which take into account thermodynamic aspects of adsorption sites and isotopic competition, is proposed to understand the enhancement of deuterium fractionation. This mechanism could play a key role in chemistry at the surface of interstellar dust grains. (C) 2005 Elsevier B.V. All rights reserved.