Several composites were prepared by mechanochemical activation of the mixture of well-crystallized LaFeO3 and CeO2 (La:Ce = 0.1, 0.8), including following calcination at 900 A degrees C, and characterized by means of XRD, XPS, HRTEM, EDX and UV-Vis DRS. Their tests as catalysts for N2O decomposition and CH4 oxidation at 700-900 A degrees C showed non-additively high activity of calcined composites in both reactions. O-16/O-18 exchange was performed at 800 A degrees C in LaFeO3, CeO2 and two composites (both calcined and non-calcined, La:Ce = 0.1) representing mainly unmodified CeO2 core covered by the finely dispersed LaFeO3, La and Fe oxides penetrating near surface layers of fluorite lattice. Namely calcined composite revealed the highest rate of oxygen transfer in the two-phase system "O (2) (gas) -surface-catalyst bulk" due to prominent increase of the rate of heteroexchange on the modified CeO2 surface at keeping unchanged the highest bulk oxygen mobility characterizing CeO2 that explained its activity in the reactions with oxygen participation.