High internal area carbon compacts have been used to measure thermo-osmotic separation factors for the mixtures C3H8 + CO2, n-C4H10 + CO2 and n-C4H10 + CH4 over ranges of initial pressures and compositions. Two procedures were employed : in the first, two vessels were connected by the thermo-osmosis membrane only; in the second, in addition to the membrane, a return capillary was provided in which isothermal flow was at least partly viscous in character. In all three gas mixtures, using either method, the more strongly adsorbed components (n-C4H10 or C3H8) were enriched at the hot side and the more weakly sorbed (CO2 or CH4) at the cold side of the membrane. In the second method the separation factors obtained with a long narrow return capillary were comparable with or larger than those measured without the return capillary. Separation factors increased the richer the mixture in the more strongly adsorbed component and the higher the initial pressure. For both methods separation factors were often large enough to indicate good potential for thermo-osmosis as a separation strategy. Heats of transport of the components in mixtures differed significantly, sometimes even in sign, from these heats for the pure components. Thus separation factors cannot be predicted from heats of transport of each pure gas or vapour. Rates of approach of separation factors to their steady state values were sensitive to the cross-sectional area of the membrane and also to stirring the gas mixtures during their un-mixing.