Ammonium molybdate, ammonium dichromate or ammonium tungstate modified zirconia are considered to be catalysts with superacid properties. The activation of these catalysts involves complex chemical changes. In the present study, combined TGA-DTA, TG-MS and TG-FTIR techniques were used to investigate the weight loss and the phase transformations of the solids and to monitor the chemical compounds that evolved under controlled heating in different environments. Through online analysis of the evolved species, some insight into the activation mechanism can be obtained. The phase transformation temperature of zirconia was shifted to a higher temperature when the sample was treated with any of these anions, which indicates that the thermal stability of the catalysts was enhanced.