A highly active superacid of 2 wt% Fe-supported ZrO2 for the skeletal isomerization of butane to isobutane was obtained by exposing Fe2O3/ZrO2 to 1 N H2SO4 followed by calcining in air at 700 degrees C for more than 24 h; the Fe2O3/ZrO2 was prepared by impregnating zirconia gel with a solution of Fe(NO3)(3) followed by drying at 300 degrees C (2 wt% Fe). A much lower activity was observed with the opposite procedure, where the first impregnation was sulfation of the gel, followed by a second impregnation with the iron compound. It was proved from analysis of the sulfur content in the catalysts that residual sulfur species were not related with generation of the superacidic sites. XPS showed the catalyst to be Fe2O3 supported on ZrO2.