The mechanism of polymer fractionation in a Clusius-Dickel thermal diffusion column was elucidated by a systematic study of the effects of important experimental factors on the separations attainable combined with an analytical description of column operation and recent theories of polymer thermal diffusion. Several polystyrene samples of known molecular weight distribution were studied in toluene and methyl ethyl ketone solvents at varying conditions of temperature gradient (up to 1050°C./cm.), mean temperature (20–45°C.), and concentration (up to 2.4 wt.-%). The thermal diffusion column was of the concentric cylinder type, equipped with thirty sample ports. The degree of separation of polymer from solvent increased with temperature gradients below 40°C./cm., increased slightly with decreased mean temperature, increased markedly with starting concentration, and was greater in toluene at a given concentration. The degree of fractionation was found to increase rapidly with temperature gradient, and was largely independent of concentration in methyl ethyl ketone but increased strongly with concentration in toluene solutions. These effects were interpreted by means of a theoretical calculation of the influence of both ordinary and thermal diffusion on column behavior in conjunction with an evaluation of the radical concentration gradients. The experimental results are in accord with a recent theory of polymer thermal diffusion which predicts that the thermal diffusion coefficient should not be a strong function of molecular weight. The fractionation effect is apparently governed by differences in ordinary diffusion coefficients. The fractionation was greater for higher molecular weights.