This paper deals with the influence of mechanical activation and decrepitation on the process of formation of active MgO in the course of the thermal decomposition of magnesite. It has been confirmed that the activation has a favourable influence on the formation of the porous oxidic product with specific surface area S-A > 100 m(2) g(-1) which comes into existence in the active zone of the decomposition of magnesium carbonate. The effect of decrepitation with secondary fragmentation on the reactant takes place at temperatures above 923 K. The process of forming active MgO was described by the KEKAM equation with a changing exponential factor.