The techniques of in situ Fourier-transform infrared spectroscopy (FT-IR) and temperature-programmed desorption (TPD) were employed to discriminate between the surface species formed over U3O8/MCM-48, bulk U3O8 and MCM-48 during the interaction of methanol at different temperatures. The U3O8/MCM-48 was found to exhibit a different behavior, as compared to the other two catalysts. Thus, methanol reacted over U3O8/MCM-48 to form methoxide, oxymethylene, polyoxymethylene (POM) and formate complex type species over catalyst surface. These species gave rise to formation of formic acid, formaldehyde, CO, CO2, H-2 and methane at elevated temperatures, as revealed by TPD coupled with QMS and FT-IR results. The room temperature adsorption of CH3OH over MCM-48, on other hand, resulted mainly in the development of methoxide species. Small amounts of dimethyl ether, CO and CO2 were the main reaction products formed during subsequent thermal activation, the yields depending upon temperature. Furthermore, a negligibly small amount of methanol was adsorbed over bulk U3O8 during room temperature adsorption and no methoxy groups were formed in the process, surface formate and oxymethylene groups were observed at elevated temperatures. The results indicate that the presence of highly dispersed U3O8 promoted the formation of certain polyoxymethylene [U(-OCH2)(n)] species that are responsible for the selective oxidation of methanol to formaldehyde and formic acid. The role of the particle size Of U3O8 crystallites and the transient surface species responsible for the selective catalytic behavior Of U3O8/MCM-48 are discussed in the light of these results. (C) 2004 Elsevier B.V. All rights reserved.