The reduction conditions of bulk molybdenum trioxide MoO3 were varied and, using n-hexane, the isomerizing properties of the resulting material examined. The more active catalysts resulted from reduction temperatures around 350 degreesC. Reduction at temperatures higher than 400 degreesC gave weak isomerization and mainly cracking processes. At temperatures lower than 300 degreesC the reduction of MoO3 became too slow to be studied. At a given temperature the use of low hydrogen partial pressures led to less active isomerizing catalysts. X-rays diffraction (XRD) patterns of these differently reduced catalysts have revealed that when MoO2 was the main reduction product - generally associated with some defect suboxide of MoO3 or Mo metal - very poor isomerization proceeded, if any. In contrast, an important isomerizing activity was regularly observed with reduced samples which contained MoOx - an up to now unidentified molybdenum oxide, but some of the present results suggest that its stoichiometry could be close to MoO. The formation dependence of MoOx on the temperature and hydrogen pressure is compatible with the existence of a molybdenum hydrogen bronze precursor whose immediate decomposition achieves, in addition, amorphization of the material. Whether the MoOx formation or the amorphization of the material is the cause of the isomerizing properties is still under discussion. (C) 2004 Elsevier B.V. All rights reserved.