The phase-boundary reaction that takes place in the near-surface of the powders plays an important role in the reactivity of fine powders. Surface reactivity of oxide powders in four solid-solid react8n systems, Fe₂O₃-ZnO, Al₂O₃-ZnO, BaCO₃-TiO₂ and Fe₂O₃-Al₂O₃, was investigated on the basis of the thermal characterization of the surface layer (< 50nm thick) of the constituent powders with different preparation histories by emanation thermal analysis (ETA) using ²²⁶Ra as parent isotope. ETA provided useful information on the phase-boundary processes taking place in the initial reaction step.