Although a huge amount of experimental work has been done, there is no clear picture about the superacidity in sulfated zirconia (SZ). Some authors think that sulfated zirconia is not superacid at all. Moreover, the structure of active site in n-butane isomerization is still now under controversy. This work tries to give some light about the mechanism of n-butane isomerization, taking into account experimental and theoretical works from others and from our own. MM2 and DFT simple calculations were performed in order to clarify the nature of the supported active species in sulfated zirconia. It is clear that a penta-coordinated species, monomeric or even dimeric, would be responsible for forming the most active sites at reaction conditions of n-butane isomerization. This idea is the key of this article. Different studies demonstrated that no superacidity is present before contact with n-butane. The sites formed at reaction conditions allow the n-butane dehydrogenation and its isomerization. A bimolecular mechanism of n-butane isomerization on penta-coordinated sites is proposed to occur. Several steps are discussed. (C) 2003 Elsevier Science B.V. All rights reserved.