The isomerization on sulphated zirconias of the 3-methyl(3-C-13)pentane, molecule chosen to distinguish between the methyl and ethyl migrations, is studied. The products distribution shows 0% for the 3-methylpentane isotoporners (not detected? or not formed?), what makes the interpretations difficult. Supposing these molecules formed in low proportions but not detected, a reaction pathway involving protonated cyclopropanes is proposed: the isotoporners are obtained from one- two- or three-step reactions. However, it is envisaged the possible non-formation of these molecules and a reaction pathway involving protonated cyclopropanes, cyclobutanes and bicyclopropanes is suggested: the C-13-label scrambling would be a two-step isomerization and the other isotopomers would be formed via a one-step reaction. The relative proportions of methyl migration (MM) and ethyl migration (EM) are assessed. The MM/EM ratio could be appreciated as an intrinsic characteristic of the catalyst because of correlations with, e.g. the crystalline structure, the sulphate content and the activity. Such correlations knowledge could help preparing catalysts with high activity and selective reactivity. (c) 2006 Elsevier B.V. All rights reserved.