This paper studies the isomerization mechanisms of 3-methyl(3-C-13)pentane over Pt supported sulphated zirconias (SZs). Pt hydrogenates the reaction intermediates that desorbs, hinders Successive isomerizations from occurring and reduces the isomerization steps to two (three with Pt-free SZs). The isomerization is a monomolecular process to which Pt does not participate. Lastly, it is suggested that the ratio between the proportions of methyl migration and ethyl migration could be appreciated as a catalyst intrinsic characteristic.