A modified Loschmidt apparatus for the measurement of the diffusion thermoeffect as a function of pressure is described. Measurements are reported for H2–Ar, H2–CO2, and H2–CH4 for pressures between 1 and 20 atm at room temperature. The experimental results have been explained semiquantitatively in terms of the pressure dependence of the relevant transport coefficients and their first composition derivatives. Terms arising because of the nonideality of the gases are also included in the phenomenological description. Relative values of diffusion coefficients for the three systems determined from the measurements are in good agreement with those determined by other methods, and the effect of pressure on the diffusion thermoeffect parallels the pressure dependence of the thermal diffusion factor as determined from gas-unmixing measurements. Suggestions are made for the future use of the method in determining other transport coefficients.