The reactions of benzene and pyridine on sulfated zirconia (I), O.6 wt%Pt/SO42-/ZrO2 (II), and 2 wt% Fe/0.5 wt% Mn/SO42-/ZrO2 (III) upon heating between 100 and 800 degrees C in an inert atmosphere were studied with a TG/MS instrument. Benzene was converted to carbon dioxide and coke, whereas most pyridine reacted in the same way, but a minor amount was desorbed unchanged. Sulfur dioxide and oxygen also evolved in all cases. Formation of sulfur dioxide indicates that sulfate is the oxidizing species. The coke was also converted to CO2 in a second heating cycle, conducted in air, but the solid gained weight in this step, indicating that some of the oxygen consumed for the oxidation of the organic molecules originated in the ZrO2. These results show that the TPD of very weak (benzene) or even stronger (pyridine) organic bases does not measure the acidity of sulfated zirconia or of composite materials based on it.