The thermal behavior of magnesium potassium phosphate cement (MKPC) during the early reaction period has been monitored by isothermal conduction calorimetry at 298 K. It is found that the typical heat evolution curve of the MKPC hydration has one endothermic valley and two distinct exothermic peaks. It is believed that the endothermic valley corresponds to the dissolution of potassium dihydrogen phosphate and the exothermic peaks are related to the dissolution of magnesia and the formation of magnesium potassium phosphate hexahydrate, respectively. The influences of the water to powder ratio, the magnesium to phosphate molar ratio and the setting retarder content on the reaction can be reflected in a change of shape and intensity of the peaks on the heat flow curve. The variation trend is consistent with the change of the setting time of MKPC pastes. © RILEM 2011.