The surface of magnesia acquires adsorptive-catalytic reactivity only after high temperature (HT) dehydration. Temperature-programmed desorption (TPD) of water reveals at least four energetically different forms of adsorbed water. The most significant HT desorption (above 1000 K) exposes low-coordinated (LC) MgO sites which are active sites for hydrogen chemisorption. Water and hydrogen TPD measurements allow the evaluation of the LC site concentration at about 7.5 x 10(17) and 7.1 x 10(17) site m-2, respectively, which corresponds to about 7% of the total ion surface population. The concentration of the LC sites confirms the much higher heterogeneity of the real MgO surface than that based on simple surface area modelling (about 2.1 x 10(17) site m-2). A dehydration mechanism for the energetically heterogeneous surface of magnesia is postulated.