A highly active solid superacid of sulfated tin oxide was prepared from tin oxide gel which was precipitated from SnCl4 solution and washed with aqueous ammonium acetate solution. The sulfated tin oxide showed activity much higher than the sulfated zirconia, a well known solid superacid, for the skeletal isomerization of n-pentane, and its acid strength estimated by temperature-programmed desorption using argon was also high er than that of the latter.