Pt-Sn/ZnAl2O4 catalysts prepared by two-step impregnation (first tin), with a Sn/Pt atomic ratio ranging from zero to 6.72, were characterized by hydrogen chemisorption, temperature-programmed reduction and tested in isobutane dehydrogenation. Metal dispersion correlates linearly with reaction rate; both parameters reach a maximum when the Sn/Pt atomic ratio is about one. The activity of the sites capable of hydrogen chemisorption, as expressed in turnover frequency number, TOF, decrease as the tin concentration is increased. From theoretical ab-inito calculations, it is proposed that tin reduces the platinum-hydro en charge transfer responsible for hydrogen dissociation through an orbital overlapping.