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 MAPbI_3–xBr_x 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.