The design and control of the surface is extremely important for the development of heterogeneous catalysts because surface properties always play a key role in catalytic performance. Therefore, it is of great interest to investigate the evolution of the surface state during the preparation of a catalyst. Mixed oxides are a particularly important group of catalytic materials. This work studied Fe2O3-MoO3 as a model system, investigating the surface states jointly influenced by the thermal spreading of MoO3 and the solid-state reaction that produces Fe-2(MoO4)(3) during heat treatment. X-ray photo-electron spectroscopy. scanning electron microscopy and Fe-57 Mossbauer analysis were used to characterize the evolution of the surface and the bulk of solids, and the oxidation of methanol to formaldehyde was also used as a probe reaction. It was found that the evolution of the surface layer takes place mainly as follows: (i) a small amount of MoO3 can be dispersed onto the surface of Fe2O3 via grinding: (ii) the thermal spreading of MoO3 and the solid-state reaction start almost simultaneously at around 400 degrees C, leading to the coexistence of MoO3 and Fe-2(MoO4)(3) species on the surface of Fe2O3 grains; (iii) further thermal spreading and the solid-state reaction yield a shell of Fe-2(MoO4)(3) encapsulating the remaining Fe2O3 grains, but a small amount of MoO3 remains on the external surface of the resulting Fe-2(MoO4)(3) shell; (iv) when the MoO3 grains run out, the surface MoO3 species also disappears. (C) 2008 Elsevier B.V. All rights reserved.