Spherical and quite monodisperse particles (average diameter 60 +/- 7 nm) of hematite (alpha-Fe2O3) were synthesized and then covered with a shell of yttrium oxide of variable thickness. A surface thermodynamic study was carried out for these core/shell colloidal particles, using two experimental techniques : contact angle measurements of selected liquids on glass slides uniformly covered by the material and determination of the penetration rates of liquids through thin layers of the solid. Using van Oss et al.＇s model of interfacial interactions (van Oss, C. J. Interfacial Forces in Aqueous Media, Marcel Dekker : New York, 1994), the surface free energy, gamma(S), of the particles was characterized in terms of its two components, Lifshitz-van der Waals, gamma(S)(LW), and acid-base, gamma(S)(AB). The latter is assumed to be the consequence of both the electron-donor, gamma(S)(-), and electron-acceptor, gamma(S)(+), characteristics of the solid surface. The efficiency of the coating of the hematite core by yttrium oxide is quantified through the comparison of gamma(S)(LW), gamma(S)(+), and gamma(S)(-) values for pure hematite and pure Y2O3, with those of the composite particles. It is found that gamma(S)(LW) does not depend significantly on the nature of the surface considered, ranging from 46 +/- 1 mJ/m(2) for pure hematite to 51 +/- 1 mJ/m(2) for pure Y2O3. Both the pure and composite particles show negligible electron-acceptor (gamma(S)(+)) character. Unlike gamma(S)(+), the electron-donor parameter, gamma(S)(-), is very sensitive to the surface composition. For composite particles ys is closer to that of pure yttria than to hematite＇s. The shell appears to efficiently hide the interfacial interactions of alpha-Fe2O3, so that the particles are, in most practical respects, essentially yttria. The implications of this fact on the stability of hematite/yttria suspensions are discussed.