Experiments were performed on magnesium oxide, using water vapor with N-2 or CO2 as carrier gases, between room temperature and 70 degrees 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 CO2 was used as carrier gas the MgO reactivity changed, producing hydrated magnesium carbonate, due to the CO2 absorption. On the basis of these results and the fact that under dry conditions CO2 is not absorbed on MgO at T<70 degrees C, a possible MgO-H2O-CO2 reaction mechanism is proposed, where MgO microstructural parameters play a significant role during the CO2 absorption. (C) 2011 Elsevier B.V. All rights reserved.