Skeletal isomerizations of n-butane and n-pentane were performed over solid superacids of sulfated zirconia and Pt-promoted sulfated zirconia. Changes in apparent activation energy and product selectivity were observed during the reactions. Isomerization proceeded by a monomolecular mechanism on Lewis acid sites in the initial period; then it changed to a bimolecular mechanism on Bronsted acid sites by forming surface alkenes. By addition of hydrocarbons or hydrogen, the monomolecular reaction became predominant, n-Pentane was converted into isopentane with very high selectivity. The overall mechanism of acid-catalyzed skeletal isomerization of alkanes is discussed.