The equations which may be employed to predict the optimum angles of inclination for enrichment of heavy water in an inclined flat-plate thermal-diffusion column with transverse sampling streams, have been derived. Considerable improvement in performance is obtainable if the column is tilted at the optimum angle, 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. Accordingly, essential savings in fixed charges and operating expense can be achieved, particularly for lower production rate operations or for smaller degree of separation.