The optimum plate spacings for maximum recovery, maximum production rate and minimum column length in a flat-plate thermal-diffusion column for the improvement of deuterium (D) from water-isotopes mixture (H2O-HDO-D2O) have been determined. The recovery of deuterium was estimated from the degree of separation of HDO and D2O. Considerable improvement in performance is obtainable if the plate spacing is suitably adjusted. The appropriate temperature differences between two vertical plates of a thermal-diffusion column for keeping the total expense fixed are also delineated. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.