The separation theory of thermal diffusion in inclined flat-plate columns of countercurrent-flow Frazier scheme is developed and investigated. The equations for the optimum angles of inclination as well as for maximum separation, maximum output and minimum column length, are derived. The author has previously shown that considerable improvement in performance is obtained when a concurrent Frazier scheme is operated at the optimum angles of inclination: it is shown here that further improvement is achieved if the feed streams are conducted in countercurrent flow.