The thermal decomposition of supergene potassium-jarosite and sodium-jarosite samples from the weathering profiles of sulfide deposits in the northern Tibetan Plateau, China, was investigated. Electron microprobe, scanning electron microscopy, and X-ray diffraction analyses indicated the presence of nearly pure potassium-jarosite and sodium-jarosite. Thermogravimetric analysis of the potassium-jarosite sample revealed mass losses of 11.39 wt% at 443.0 °C, 20.99 wt% at 688.3 °C, and 3.18 wt% at 779.3 °C. The thermogravimetric analysis of sodium-jarosite revealed mass losses of 11.72 wt% at 447.5 °C, 21.32 wt% at 682.6 °C, and 3.70 wt% at 716.5 °C. The results provide no evidence for water-molecule loss below 400 °C, as has been reported previously for natural potassium-jarosite and sodium-jarosite. Thermal-decomposition mechanisms have been proposed for potassium-jarosite and sodium-jarosite based on X-ray diffraction analyses of samples obtained at distinct points along the respective thermal decomposition processes. A comparison of the thermal analysis patterns of potassium-jarosite and sodium-jarosite indicates that sodium-jarosite undergoes the initiation of lattice destruction at a higher temperature.