The effects of the reduction process, under a hydrogen flow, of a bulk WO3 tungsten oxide powder on the reactivity at 350 degrees C of 2-methylpentane (2MP) and 4-methyl-1-pentene (4M1Pene) were investigated. The best experimental conditions to obtain a stable state exhibiting interesting skeletal isomerization properties were determined. Catalyst characterization by XPS, XRD, TPR, and surface area measurements allowed us to propose some intermediates phases and the corresponding nature of active sites formed during the reduction process of the WO3 tungsten oxide. Correlations between these catalyst characterizations and the catalytic properties show that the WO3 phase is initially inactive for both alkanes and alkenes reactions. When the W20O58 phase is formed, which corresponds to the first reduction step of WO3, the surface becomes acidic in character, leading to the skeletal isomerization of alkenes by a monofunctional acidic mechanism. The reduction of the W20O58 tungsten oxide occurs in two parallel ways, to WO2 and W3O, When this reduction is performed at moderate temperature (350 degrees C), a stable surface composed of these two phases (WO2 and W3O) is obtained. This surface is active and very selective for the skeletal isomerization reactions of alkanes and alkenes, This catalytic behavior has been interpreted with the aid of two mechanisms: a bifunctional one on the WO2 phase and a monofunctional metallacycle one on the W3O phase. (C) 2000 Academic Press.