The modelling of the chemical transformation of powder systems is known to involve nucleation and growth processes, which occur in different zones of the solid particles. When these two processes are in competition, the overall rate depends both of the geometry of the particles, and of the intrinsic reactivity of the powder that can be characterised due to the determination of the surfacic nucleation frequency and reactivity of growth. Several examples are used to illustrate it from the comparison between the experimental kinetic curves and those deduced from models of nucleation and anisotropic or isotropic growth.