Forces between unlike molecules are investigated via mutual and thermal diffusion measurements on4He‐85Kr andH2‐85Kr. New experimental results are reported, the thermal diffusion measurements covering the temperature range 77 to 760°K. Above 300°K the measurements are in reasonable accord with those of Ghozlan and Kistemaker, but below 300°K there are substantial discrepancies. Evidence is presented which indicates that the previous results at the lowest temperatures, particularly for He‐Kr, are incorrect. Present measurements support the adequacy of the common potential parameter combination rules forH2‐Kr, but not for He‐Kr; in the latter case the potential well depth &egr;12is apparently smaller than theoretically predicted, a conclusion in accordance with molecular‐beam scattering measurements.