The mobilities of Rb+ ions in Kr and Xe gases have been measured at 300°K in a drift tube mass spectrometer over a wide range of the ionic energy parameter E/N (the ratio of the electric field intensity to the neutral gas number density). The Viehland–Mason kinetic theory of ionic mobility (valid for arbitrary E/N) is used to derive theoretical mobilities from the electron gas-Drude model potentials of Gordon and Waldman for Rb+–Ar (mobilities measured previously), Rb+–Kr, and Rb+–Xe. The results are compared with the experimental data. Further, by using an iterative technique, new potentials are developed which are determined directly by the experimental data, and these ’’experimental’’ potentials are tabulated and compared with the Gordon–Waldman potentials.