The present study deals with the estimation of deuterium recovery from the separation of water-isotope mixture (H(2)O-HDO-D(2)O) by continuous-flow thermal diffusion. First, the equations for predicting the degrees of separation for each component in H(2)O-HDO-D(2)O system were derived. The recovery of deuterium (D) was then estimated from the degrees of separation of HDO and D(2)O. The most important assumption in this work was that the concentrations of each component were at local equilibrium within the thermal diffusion column, i.e. H(2)O + D(2)O reversible arrow 2HDO. It was found that D(2)O preferably transfers to the bottom end of the column, while HDO does the same for dilute feed concentration, but then transfers to the top of the column when the feed concentration of D(2)O, C3F, increases to 0.3. The maximum total recovery of deuterium from HDO and D(2)O occurs at C(3F) = 0.25. (c) 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.