In situ synchrotron X-ray diffraction determination of product particle size during the thermal decomposition of CdCO₃ powders is evaluated for sintering kinetics. The rate-determining step for particle growth is not the same during the reaction as after, the data suggesting surface diffusion during decomposition but evaporation-condensation after. Reasons for the difference are discussed, including the parent-phase matrix and CO₂ from the decomposition. When compared with loss of crystal imperfections, four stages of growth are seen, each affecting the reactivity of the product.