The simple but precise equations of separation applicable to the whole range of concentration in various types of improved thermal diffusion columns have been derived in a common form. The optimal conditions for the whole range of concentration are exactly the same as those for the equifraction solution, when 0.3 < c < 0.7, while the calculation of maximum separation must be modified. Considerable improvements in separation for the whole range of concentration can be obtained by employing the improved columns, operated at the optimal conditions, instead of using the Clusius-Dickel column.