Modulated differential scanning calorimetry (MDSC), thermogravimetric analysis (TGA) and X-ray diffraction were used to study the phase transitions in single crystals of KHSO4 (KHS) above room temperature. Our results show three phase transitions, on heating a freshly prepared sample: the first one at ≈︂120 °C (exothermic and metastable) accompanied by small weight loss (less than 0.25wt%), the second one at ≈︂178 °C (endothermic) and the third one at ≈︂212 °C (endothermic, melting point). The latter two phase transitions are also accompanied by weight loss and its rate depends on the surface of the sample. We discuss the first transition in terms of absorbed water molecules weakly bound to the surface that are released at 120 °C on heating a fresh crystal from room temperature. The second transition at 178 °C is discussed in terms of a partial polymerization setting in at reaction sites distributed on the surface of the crystals followed by the final decomposition above 212 °C.