Nanoparticle iron oxide catalyst was studied to determine the phase changes that occur during catalysis experiments and to determine if these changes could explain the oxidative catalysis and deactivation mechanisms. The starting material was characterized as a mixture of glassy material, poorly crystalline iron hydroxides, and a small amount of highly crystalline gamma-Fe2O3. Under oxidative (with respect to phase changes in iron oxide) heating conditions (3% oxygen and higher) this material transformed to a much coarser alpha-Fe2O3, gamma-Fe2O3, or Fe3O4 phase depending on the conditions of the experiment. When the material was pre-heated to intentionally transform it to alpha-Fe2O3, and then later exposed to catechol vapors below 450degreesC, the alpha-Fe2O3 was further transformed to either an oxygen deficient gamma-phase or to magnetite even though input oxygen tension remained well above the magnetite (Fe3O4), hematite (alpha-Fe2O3) phase boundary. These two phases (gamma-Fe2O3 and magnetite), both of which are spinels, do not seem to be catalytically active for either the oxidation of CO or the oxidative degradation of catechol. It is proposed that the appearance of these phases are evidence of an intermediate reduction step in the oxidative catalysis mechanism performed by iron oxide. Specifically, this provides ex situ analytical evidence that a mechanism similar to the Mars-van-Krevelan mechanism operated for the catalysis by bulk crystalline iron oxide. Subsequent oxidation of the reduced iron oxide would normally regenerate the alpha-phase completing the Mars-van-Krevelan cycle. However, in cases where the rate of reduction exceeds the rate of oxidation, the iron oxide permanently transforms to a spinel and catalysis halts even though one of these spinels, maghemite (gamma-Fe2O3), has no stability field in P-T space with respect to magnetite and hematite. The appearance of magnetite is similar to the results expected for reducing conditions and further attests to the role of alpha-Fe2O3 as an oxygen source during catalysis rather than as an electron acceptor. (C) 2003 Elsevier B.V. All rights reserved.