Density functional theory was used to investigate the chemical reactivity of oxygen vacancies on the MgO surface towards CO2, NO2 and metal atoms. The oxygen vacancies induce changes in the electronic structure of the MgO compared to that of the regular surface. The most common surface defects can have one or two electrons confined in the vacancy. The main effect of the vacancies is the reduction in the energy of the gap between the HOMO and LUMO of the MgO causing in this way a change in its reactivity towards adsorbed molecules such as CO2 and NO2 or atoms such as Ni, Pd, Pt, Cu and Zn. It was found that F-s centers are more reactive than F-s(+) centers. In the case of metal atoms, the level of interaction is proportional to the degree of transfer of electron density towards the incoming atom. (C) 2008 Elsevier B.V. All rights reserved.