CaO hydration is an exothermic reaction that can be used to supply heat. The amount of heat released by the reaction is dependent on the operating steam pressure. In this work, the rate of CaO hydration was measured with a high-pressure thermogravimetric apparatus under steam pressures in the range of 0.67-3.8 MPa. At these pressures, CaO hydration occurred at high temperatures up to 1023 K. The CaO hydration rate varied by the power of 2, with respect to the difference between the reactant steam pressure and the equilibrium steam pressure (P-H2O -P*(H2O)). The apparent activation energy for reaction constant k was determined to be 8.4 kJ/mol. The particle size (d(p)) slightly affected the CaO hydration rate. The hydration rate ( R) is given by R) 1/d(p)(0.11) x 0.0069 exp[- 8400/(RT)] x (P-H2O - P*(H2O))(2).