The effects of thermal treatments under vacuum, used as a way to generate reduced centers on Rh2O3 and RhOx/CeO2, have been studied by ESR and FTIR, using respectively oxygen and carbon monoxide as probe molecules. The results obtained for the outgassed samples reveal the presence of ceria-rhodia interactions favoring the stabilization of paramagnetic Rh2+ cations in rhodium oxide clusters on the ceria surface. Subsequent O-2 adsorption leads to the formation of different oxygen-related paramagnetic species located on ceria, on rhodium oxide clusters and at the boundary between both oxides; their contribution to the spectra depends on outgassing conditions and O-2 adsorption temperature, The unexpected absence of O-2(-)-Ce4+ species after O-2 contact at 77 K with RhOx/CeO2 outgassed above 573 K evidences the existence of electronic interactions between the RhOx and CeO2 phases, being explained on the basis of electron transfer to the mixed valence RhOx phase from the surface-reduced ceria, leading to electron depletion of the latter. This effect is inhibited by CO adsorption, showing the dependence between the electron-accepting properties of the rhodia clusters and the presence of vacant coordination sites at the surface Rh ions, An effect of similar kind may be responsible for shifts observed in the IR bands of rhodium dicarbonyls formed in the RhOx/CeO2 system. The latter results suggest the possibility that thermal enhancement of surface reactions in complex systems could depend on electron transfer between adjacent phases and that adsorption on one phase may influence the surface reactivity of another phase by affecting to the electron transfer between them.