n-butane isomerization on sulfated zirconia was studied in the temperature range 393-473 K. Rapid deactivation occurs, when the reaction is carried out in He carrier gas. In contrast, stable stationary activity was observed in H-2 carrier gas. In situ Raman and UV-VIS diffuse reflectance spectroscopy and ESR showed that the deactivation is caused by the formation of allylic and polyenylic cations and polycyclic aromatic compounds, the formation of which is largely prevented in the presence of H-2. Deactivated catalysts can be fully regenerated by treatment at 723-753 K in flowing O-2. The regeneration process was also followed by in situ spectroscopies.