Measurements of thermal diffusion were made with one side of the apparatus at about 15° C. and with the other at temperatures down to about - 190° C. For mixtures such as H2-CO2, H2-N2O, H2-C2H4, where one constituent easily liquefies, the separation is proportional to log (T1/T2) throughout the whole range investigated, where T1 is the absolute temperature of the hot side and T2 that of the cold side. For mixtures such as H2-N2, H2-CO, H2-A, which can be examined at lower temperatures, the separation is at first proportional to log (T1/T2), and then falls below the proportional value. In mixtures of N2-A this falling off at low temperatures is more pronounced. From these measurements values of kt, the ratio of the coefficient of thermal diffusion to the coefficient of ordinary diffusion, can be obtained over a considerable range of temperature. It is suggested that the behaviour at low temperatures may be explained by the fact that real molecules cannot be regarded as point centres of repulsive force only. The general behaviour of a mixture of H2-O2 is similar to that of H2-N2 or H2-A. It is found that thermal diffusion is unaffected by reduction of the pressure to about one-fourth of an atmosphere. This result is in agreement with theoretical prediction.