The thermal stability and reactivity of adsorbed pyridine on sulfated zirconia and Pt/sulfated zirconia were studied by temperature-programmed desorption (TPD) coupled with infrared spectroscopic and mass spectrometric analyses. Adsorption of pyridine on sulfated zirconia produced two types of adsorbed species: a pyridinium ion on the Bronsted acid site (Pyr-B) giving rise to infrared bands at 1638, 1611, 1486, and 1540 cm(-1) and a covalently bound species on Lewis acid sites (Pyr-L), giving Characteristic bands at 1486 and 1445 cm(-1) at the same rate, accompanied by the desorption of sulfates to produce Gaseous SO3. TPD studies showed that Pyr-L nearly completely decomposed to CO2 at 773-823 K, whereas more than 60% Pyr-B remained on the surface in the same temperature range, indicating that Pyr-B possesses higher thermal stability than Pyr-L. The addition of Pt to sulfated zirconia lowers the decomposition temperature of sulfate as SO2. The study shows that that temperature -programmed desorption/decomposition with simultaneous infrared spectroscopic and mass spectrometric analyses is an effective approach for identifying the structure of the adsorbed species leading to decomposed products. TPD of adsorbed pyridine is not a reliable approach for measuring acid strength of the acid/base catalysts. (C) 2003 Elsevier B.V. All rights reserved.