Lithium chloride-modified magnesium hydroxide is a candidate material for thermochemical energy storage. In this work, the effects of lithium chloride mixing ratio, hydration temperature, and water vapor pressure on the hydration behavior of the material are investigated. Heat output densities for all experimental conditions are evaluated. The heat output density per unit weight of lithium chloride-modified magnesium hydroxide, at a molar mixing ratio of 0.10 mol of lithium chloride per mol of magnesium hydroxide, is 1.40 x 10(3) kJ kg(-1) at a hydration temperature of 110 degrees C and a water vapor pressure of 57.8 kPa. This value is higher than the heat output density of authentic magnesium hydroxide.