Mobilities of Br− ions in Ar, Kr, and Xe gases at 300 K have been measured in a drift tube mass spectrometer over a wide range of E/N (the ratio of the electric field to the neutral gas number density). For any ion–neutral gas interaction potential, implementation of the Viehland–Mason kinetic theory (accurate for arbitrary E/N) yields theoretical mobility data. This theory is employed in conjunction with an iterative-inversion scheme which uses experimental mobility data to modify an initial ‘‘guess’’ 4,6,12 potential to produce a ‘‘directly determined’’ potential which accurately reproduces the experimental mobilities for a given ion–neutral atom pair. These potentials are tabulated for Br−–Ar, Br−–Kr, and Br−–Xe and their salient features are compared to previous ‘‘directly derived’’ potentials of other ions in these gases.