A separation theory for the recovery of deuterium from water-isotope mixture in flat-plate thermal diffusion columns of the Frazier scheme inclined for improved performance has been developed. Equations for the optimal angles of inclination and the corresponding maximum recovery, maximum production rate and minimum column length have been derived. Considerable improvements in performance are obtainable if the columns are inclined at the best angles, so that the convective strength can be properly reduced and controlled, resulting in suppression of the undesirable remixing effect while still reserving the desirable cascading effect. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.