The optimal angles of inclination for maximum recovery, maximum production rate and minimum column length in a continuous-type flat-plate thermal diffusion column inclined for the improvement in recovery of deuterium (D) from water-isotopes mixture (H2O-HDO-D2O), have been determined. The recovery of deuterium was estimated from the degrees of separation of HDO and D2O. Considerable improvement in performance is obtainable if the column is tilted at the optimal angle of inclination, instead of being placed vertically, so that the convection strength can be properly reduced and controlled, resulting in suppressing the undesirable remixing effect while still preserving the desirable cascading effect. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.