A separation theory for the enrichment of heavy water in flat-plate thermal diffusion columns of the Frazier scheme inclined for improved performance has been developed. Equations for the best angle of inclination and maximum separation have been derived. Considerable improvement in separation is obtainable if the columns are inclined at the best angle, so that the convective strength can be properly reduced and controlled, resulting in suppression of the undesirable remixing effect while still preserving the desirable cascading effect.