The dynamic behavior of adsorbed pyridine and the sulfate group has been studied over sulfated zirconia (SZ) and Pt-promoted sulfated zirconia (Pt/SZ) by in situ infrared (IR) spectroscopy coupled with mass spectrometry (MS). IR analysis confirmed that pretreating SZ in flowing He at 500degreesC led to the formation of S=O species. Pyridine adsorption caused desorption of sulfur in the form Of SO3- IR and MS analyses coupled with a temperature-programmed desorption (TPD) study confirmed that pyridine adsorbed onto the Lewis acid sites was oxidized to CO2 while the pyridine-Bronsted acid site complexes showed little desorption or oxidation. Addition of Pt onto sulfated zirconia led to enhanced Bronsted acidity when treated with 142; higher loading of Pt led to decreased thermal stability of the sulfate group, promoting desorption Of SO2 during the TPD. This is the first report that presents dynamic analyses of pyridine adsorption/desorption phenomena over sulfated zirconia samples. (C) 2003 Elsevier B.V. All rights reserved.