The separation theory of thermal diffusion in wired concentric-tube columns of countercurrent-flow Frazier scheme is developed and investigated. The equations for the optimum wire angles of inclination as well as for maximum separation, maximum output and minimum column height are derived. The author has previously shown that considerable improvement in performance is obtained when a cocurrent Frazier scheme is operated at the optimum wire angles of inclination: it is shown here that further improvement is achieved if the feed streams are conducted in countercurrent flow. (c) 2006 Elsevier B.V. All rights reserved.