Chemical and structural transformations have long been carried out by milling. Such mechanochemical steps are now ubiquitous in a number of industries (such as the pharmaceutical, chemical and metallurgical industries), and are emerging as excellent environmentally friendly alternatives to solution-based syntheses. However, mechanochemical transformations are typically difficult to monitor in real time, which leaves a large gap in the mechanistic understanding required for their development. We now report the real-time study of mechanochemical transformations in a ball mill by means of in situ diffraction of high-energy synchrotron X-rays. Focusing on the mechanosynthesis of metal-organic frameworks, we have directly monitored reaction profiles, the formation of intermediates, and interconversions of framework topologies. Our results reveal that mechanochemistry is highly dynamic, with reaction rates comparable to or greater than those in solution. The technique also enabled us to probe directly how catalytic additives recently introduced in the mechanosynthesis of metal-organic frameworks, such as organic liquids or ionic species, change the reactivity pathways and kinetics.