The microstructure and the structure of NiO-SnO2 and Fe2O3-SnO2 were studied by X-ray diffraction (XRD) and adsorption analysis (BET). The segregation of Ni or Fe on the surface of SnO2 was observed even in specimens with 30 mol% addition. The segregation caused a decrease in the surface energy of the powders and, therefore, an increase in the surface area as expected by the Ostwald ripening model. After the saturation of the tin oxide surface, there is no increase in the surface area, but a decrease due to second phase nucleation. Adding more Ni to the system, similar segregation of Sn on the surface of NiO as well as the effect on the surface area were observed. In Fe2O3-SnO2, an iron phase other than hematite was detected at high concentrations of Fe (Fe2O3-20% SnO2), suggesting the formation of magnetite or a ferrite spinel. Gibbs energy diagrams that could be used to predict stable phases of certain compositions were drawn for both systems based on the observed data.