The acidity of several types of catalyst was measured by means of in situ TG, using the procedure of NH3 adsorption-desorption, with a modified quartz furnace tube on a Du Pont 9900 Computer/Thermal Analysis System. HY and HZSM-5 zeolites were used as reference samples to test the repeatability and/or precision of this method. The total acid amount and acid strength of samples could be directly calculated from the mass change on TG-DTG curves of NH3 adsorption-desorption. The relative standard deviation of the data which were collected from five measurements of HY is less than +/-0.050. This result was examined using HZSM-5 and the data were comparable with those reported in the literature. The relations between the acidities and the catalytic activities for Pt/MgO-cross linked montmorillonite (Pt/MgO-CLM) and beta zeolite catalysts are also discussed.