This work investigates the kinetics of the anatase-to-rutile phase transformation of TiO2 powders during ball milling. Two elementary stages are observed, namely the transformation of anatase to the TiO2 II phase and of this latter to rutile. Such consecutive reactions were studied under inelastic impact regimes. Based on a suitable modeling of transformation kinetics, the fraction of powder processed on average at each collision was estimated. The mass of powder involved in phase transformation and microstructural refinement processes at each collision was then worked out by a systematic variation of the powder charge inside the reactor