The correlation between the cracking and skeletal isomerization of some hexenes and heptenes as a function of reduction time and temperature and the distribution of Mo and W surface species (determined by X-ray photoemission spectroscopy (XPS)), have been carried out on a mixed WO3-MOO3/alpha -Al2O3 sample, prepared by co-impregnation of ammonium metatungstate and ammonium paramolybdate. The amounts of each precursor were calculated in order to deposit a theoretical monolayer with the same proportion Of WO3(50%) and MoO3(50%). The use of different kind of hexenes and heptenes as probe molecules, varying by their structure, and of a saturated reactant, the 2-methylpentane, allowed us to postulate a monofunctional acidic mechanism, in which both cracking and skeletal isomerization occur on acidic sites via surface alkoxy intermediates. From the correlation with XPS measurements, it can be concluded that Mo(V) species, acting mainly as Bronsted acidic center through surface O-H groups, are responsible for cracking and isomerization reactions. Nevertheless, the presence of acidic Lewis center, like Mo(VI), W(VI) or M(V) coordinately unsaturated sites cannot be excluded. The dual acid-basic nature of O-H surface groups is pointed out.