Heavy water (D2O) is the most feasible moderator and coolant in nuclear-fission reactors while deuterium (D) will be the nuclear fuel for thermonuclear fusion in the distant future. The recoveries of deuterium in D2O and HDO from the separation of water-isotope mixture (H2OHDO-D2O) by thermal diffusion has been estimated. First, the degrees of separation for each component of water-isotope mixture in thermal diffusion columns were predicted, then the recoveries of deuterium in each component were estimated from the molecule weights. It was found that the maximum recovery of deuterium might be obtained when the feed concentration of D2O is 0.22 mass fraction. (C) 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.