[en] The role of near-surface on the reactivity of metal oxide powders in solid-solid reactions increases with a decrease in particle size. Emanation thermal analysis (ETA) using a surface impregnation method with ²²⁶Ra parent isotope was applied to the investigation of the thermal characterization of the near-surface of various inorganic powders with different preparation histories. Further, in order to consider the surface reactivity of these materials, the thermal behaviour of the labelled species in solid-solid mixed systems were were followed by means of the ETA under the working state at high temperatures. The ETA results for single powders, Al₂O₃ and Fe₂O₃, and mixed systems, Fe₂O₃-ZnO, BaCO₃-TiO₂ and Fe₂O₃-Al₂O₃, were represented as typical examples. ETA provided useful informations on the surface characterization of oxide powders, and on the phase-boundary processes taking place in the initial reaction step for the mixed systems. (author) 25 refs., 11 figs., 1 tab