ZrO2-supported Fe oxide catalysts (Fe/ZrO2) exhibit a much higher activity in the NO-CO reaction at 523 K than their Al2O3- or SiO2-supported counterparts, in particular at a low Fe content. Physicochemical characterizations of Fe/ZrO2 with varying Fe loading calcined at 973 K were conducted to disclose the Fe oxide ZrO2 interaction modes and the catalytically active species for the NO-CO reaction by means of Fe-57 Mossbauer spectroscopy, IR, XRD, XPS, and EXAFS. Three kinds of Fe3+ species were shown to be formed; two highly dispersed species and alpha-Fe2O3 particles. Fe3+ cation species are preferentially formed below 2.8 Fe nm(-2) by ion-exchange with the surface OH groups of ZrO2. After the formation of Fe3+ cation species, Fe3+ oxide clusters are formed in a maximum amount of 3.4 Fe nm-2. The Fe3+ oxide clusters are responsible for the high activity of Fe/ZrO2, whereas the Fe3+ cation species are almost inactive. The addition of Cu improves the activity of Fe/ZrO2 more than ten times. The catalytic synergy between Fe and Cu is attributed to a cooperative involvement of Cu and Fe. (C) 2000 Academic Press.