The reactions of n-butane and isobutane have been studied in a flow system in the presence of hydrogen at 250 degrees C using sulfuric acid treated zirconia as catalyst. The conversion of the butanes occurs with high selectivity in isomerization. The branched isomer reacts much faster than the linear one. The use of C-13 labelled starting material and analysis of the isotope distribution in the products show that the rearrangement is of intramolecular nature in accord with the superacidic properties of the catalyst.