Ionic liquids, such as BMIMBF4, are added to mixed halide perovskites to prevent halide phase segregation and increase phase stability, but exact mechanisms changing halide kinetics are currently unclear. Here, X-ray diffraction, nuclear magnetic resonance, and photoluminescence spectroscopy are used in situ under dark conditions to follow thermally driven halide mixing processes forming MAPbI3–xBrx from physical mixtures of MAPbI3 and MAPbBr3 powders with and without BMIMBF4. Halide migration is significantly accelerated with BMIMBF4 compared to additive-free mixtures. This is attributed to liquid-like dynamics of BMIMBF4 at elevated temperatures, liberating defect sites at perovskite interfaces. Furthermore, the presence of BMIMBF4 increases the activation energies for bromide migration, suggesting a changed nature of the latter. This is explained by a preferred interaction between BMIM+ and bromide, indicating that the cations of the additive shuttle bromide ions between interfaces. Overall, these observations pave the way for a better understanding of halide transport in hybrid perovskites.