In the preparation of active SO4-ZrO2 catalysts, several steps involving various chemical and/or physical processes are necessary. In particular it has been reported that, after sulfation of amorphous Zr hydrates, a calcination at T-calc > 773 K is needed to guarantee the crystallization of ZrO2 in the tetragonal phase. By the use of a stabilized tetragonal ZrO2, it is here demonstrated that a calcination at T-calc > 773 K is indeed necessary for all SO4-ZrO2 systems, and that its actual role is the selective elimination of sulfates from highly energetic crystallographic defects. The calcination step at T-calc > 773 K so creates the conditions for the formation of strong Lewis acid centres, that are necessary in the catalytic process, and the presence of which is here monitored spectroscopically by the reversible adsorption of carbonmonoxide.