Halide migration limits the stability of hybrid halide perovskites for optoelectronic applications, yet can be decelerated at room temperature by adding ionic liquids. An approach is presented to quantitatively evaluate the diffusion of I⁻ and Br⁻ to form MAPbI_xBr_3-x from neat perovskite powders. First, in situ X-ray diffraction (XRD) data are used to extract the time-dependent MAPbI_xBr_3-x composition of an initially physical mixture of neat MAPbI₃ and MAPbBr₃ grains. Next, an effective interdiffusion model is expanded to obtain time-dependent diffusion coefficients. They reduce with time due to the decreasing concentration gradient during the halide exchange process. In samples without any additive, Br⁻ is found to diffuse up to three times faster into the MAPbI₃ grains than I⁻ into the MAPbBr₃ grains, while the addition of the ionic liquid 1-Butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄) accelerates and nearly equalizes the interdiffusion. Analyzing the temperature dependence of the diffusion coefficient suggests that, without ionic liquid, the diffusivities are limited by the halide vacancy formation. In contrast, in the presence of the ionic liquid, halide vacancy formation is facilitated, yet is controlled by the thermal activation of the BMIM⁺ mobility.