AbstractUnderstanding the kinetics and energetics of metal halide perovskite formation, particularly from the structural point of view at the nanoscale, is important for the advancement of perovskite devices. In particular, insight is needed regarding the mechanisms by which perovskite conversion reactions occur, and their kinetics. Here, we examine the structural evolution of precursor and perovskite phases using in situ synchrotron x-ray scattering. This approach mitigates issues associated with illumination and electron beam-based techniques and allows conclusions to be drawn regarding the kinetics of these reactions. We find that kinetics and grain orientation strongly depend on both the lead halide framework and the nature of the A-cation, with fastest kinetics for MAPbI₃, followed by FAPbI₃, and slowest for MAPbBr₃. Molecular dynamics simulations and density functional theory calculations further reveal that these reactions are diffusion-controlled with a hopping time of 5-400 s, corroborating experimental findings.