In situ characterization by XPS and UPS of bulk commercial MoO3 and the equivalent five atomic layers of MoO3 deposited on TiO2 before and following the reduction by hydrogen at different temperatures led to the identification of the chemical species on the surface. These spectra reveal that MoO3 is the only state present prior to reduction. However, a predominant Mo5+ state on the sample surface is formed at reduction temperatures between 573 and 623 K. Continued reduction leads to the formation of MoO2, which reaches a stable state at temperatures between 653 and 673 K. The metallic character of MoO2 is observed as a density of states (DOS) at the Fermi level. This metallic function dissociates hydrogen molecules to atoms. Bonding of these active (H) atoms with surface oxygen leads to the formation of Bronsted acid (Mo-OH) group(s) as characterized by O 1s and catalytic properties. As a result, a bifunctional MoO2(H-x)(ac) phase is formed on the outermost sample surface layer in these systems. More sensitive UPS measurements established the presence of MoO2 in the upper four to five surface monoatomic layers during the reduction process. Combination of XPS-UPS provides valuable information on the chemical composition of the upper surface layers. Apparently, a stable sandwich-like structure is formed in which the composition from bulk to surface consists of MoO3, Mo2O5 and MoO2. The outermost surface layer in this structure is a bifunctional MoO2(H-x)(ac) phase. The catalytic performances of the different Mo suboxides towards the hydroisomerization reactions of 1-pentene and n-pentane were studied. The MoO3 phase has no catalytic activity due to its insulating properties. Double-bond and skeletal isomerization reactions of 1-pentene take place on the acid function(s) of Mo5+ state, while n-pentane and isopentane are the only products observed in the case of MoO2(H-x)(ac.) Both metallic and acid functions are required for the isomerization of n-pentane to isopentane. Consequently, it is concluded that the dehydrogenation process is the rate-determining step in the isomerization reaction of n-pentane. Hydrogenation of I-pentene is the dominant reaction when the bifunctional MoO2(H-x)(ac) phase is used. (c) 2005 Elsevier B.V. All rights reserved.