Zinc promoted H-ZSM-5 catalysts were prepared by ion exchange (Zn-e) and by impregnation (Zn-i). The acidity of these samples was characterized by the TPD of ammonia and by FTIR spectroscopy, recording the OH vibration spectra and the spectra of adsorbed pyridine and ammonia. The method of preparation had minor influence on the activity in the conversion of propane over samples containing less than 1.5 wt.-% Zn. However, Zn-e samples with Zn contents of 1.5 wt.-% and more were much more active than Zn, samples. The former displayed an increased number of Lewis sites and a decreased number of Bronsted sites. Otherwise, partial pore blocking with increasing zinc loading seems to limit the promoting effect in case of Zn-i samples. The results demonstrate that ionic zinc species determine the catalytic properties, acting as strongly acidic Lewis sites.