Experiments were performed on magnesium oxide, using water vapor with N 2 or CO 2 as carrier gases, between room temperature and 70°C, in order to elucidate its thermal stability. Initial experiments were performed with water vapor in the presence of N 2 to elucidate the different physicochemical processes produced by water vapor exclusively, as N 2 is an inert gas. These results showed that water is only adsorbed on the MgO surface. On the other hand, when CO 2 was used as carrier gas the MgO reactivity changed, producing hydrated magnesium carbonate, due to the CO 2 absorption. On the basis of these results and the fact that under dry conditions CO 2 is not absorbed on MgO at T<70°C, a possible MgO–H 2 O–CO 2 reaction mechanism is proposed, where MgO microstructural parameters play a significant role during the CO 2 absorption.