The optimum conditions in operating a thermal diffusion column have been searched using argon gas. The results obtained were compared with the theoretical values. Two molecular models used for the comparison with the experimental results are the inverse power model and the Lennard‐Jones (12–6) potential model. The former showed a little better agreement with our results than the latter. WhenP4is plotted as abscissa andP2/lnqas ordinate, the relationP2/lnq= (1/a) (P4+b) gives a straight line using the assumption given by Furryet al. Substituting the experimental results in this equation, the time‐course variation of the values ``1/a'' and ``b/a'' from the slope and intercept of these linear plots was obtained. The variation ofaandbwith time is useful as a clue to see if the system is in equilibrium or not. Minimum values were observed near the optimum pressure in each operating time. Theoretical calculation of the relaxationtrwhich characterizes the approach to equilibrium was 1.04 h. Employment of direct current to heat the wire was a little better than the alternating current.