The problem of oxide-oxide interaction has been studied on the Fe2O3/ZrO2 system reduced and outgassed under vacuum between 500 and 773 K. The techniques utilized were UV-vis reflectance, volumetric, TEM, and mainly FTIR spectroscopy of adsorbed CO. Whereas CO is not adsorbed on oxidized samples, the depletion under vacuum of oxygen from the sample reduced at 500 K produces a coordinative unsaturation (cus) on the ferric surface ions with lowering of the oxidation number and reversible adsorption of CO at v = 2150, 2138, and 2124 cm(-1). The reduction at 623-773 K favors further reduction to a highly dispersed FeO-like phase spread on the support; the frequencies of reversibly adsorbed CO are red-shifted up to 2080 cm(-1) and are the fingerprints of a high heterogeneity of Fe-x+ (x approximate to 2) sites. However, the oxide-oxide interaction hinders an easy reduction to Fe-0 even at 773 K; at this temperature only small quantities of carbonylic clusters and of CO adsorbed on more extended iron faces, with vCO = 2100-1970 cm(-1), are formed at room temperature. The oxidation at 473 K fully eliminates the process of reduction and of oxide-oxide interaction.