Since its discovery, sulfated zirconia and metal-doped sulfated zirconia have been the subject of numerous reports on their catalytic activity, Their ability to perform low-temperature hydrocarbon isomerizations has led to claims that sulfated zirconia and metal-doped sulfated zirconia are superacids or at least very strong acids. This has led to many investigations on the acid strength of sulfated zirconia and metal-doped sulfated zirconia producing varying results, We report the use of cal-ad (which uses combined information from calorimetry and adsorption of pyridine onto the solid acid) to determine the acidity of sulfated zirconia and metal-doped sulfated zirconia. We find that sulfated zirconia has two types of acid sites : 24 mu mol of a strong site, which has a strength of 31.2 kcal/mol, and 52 mu mol of a weaker site of 25.8 kcal/mol, which places the acidity of sulfated zirconia lower than that of HZSM-5 (41 kcal/mol) and about the same as that of HY (34 kcal/mol), Doping sulfated zirconia with 0.2 wt % Pt does not change the acidity, but doping with iron and manganese (which increases the catalytic activity) results in a lower measured acidify, indicating that iron and manganese have occupied the strongest acid sites.