The catalytic activities of 4-methyl-1-pentene and of the corresponding alkane, 2-methylpentane, are compared on two series of reduced MoO3 catalysts supported on alpha-type-Al2O3, prepared either by impregnation with Na2MoO4 or by the slurry impregnation method starting from MoO3. The reduction of these samples depends on the molybdenum oxide content and therefore on the interaction with the alumina surface. The use of probe molecules, such as 3,3-dimethylbut-1-ene (dibranched containing a quaternary carbon atom), 2,3-dimethylbut-2-ene (dibranched with two tertiary carbon atoms), 4-methyl-1-pentene (monobranched with a tertiary carbon atom), l-hexene(linear), and cyclohexene (cyclic), combined with C-13 tracer studies leads to the conclusion that isomerization mainly occurs on acidic sites via surface alkoxy intermediates, resulting in protonated cyclopropane species. From the correlation with XPS measurements, it can be concluded that Mo(V) species are responsible for acidic isomerization and that Mo(IV) and Mo(II) species exhibit metallic dehydrogenation properties. Nevertheless, metallic isomerization via metallocyclobutane intermediates should be considered when reduction at high temperature occurs. (C) 2000 Academic Press.