Two major chemical processes, acidobasic and redox, monitor the adsorption mechanism on metal oxides. Cations and anions of surfaces can be described as acid-base pairs. Thus, electron-rich molecules (Lewis bases) or anionic fragments formally arising from a heterolytic bond cleavage of molecule interact on the metal Mn+, while electron-poor ones (Lewis acids) interact on the oxygen one O2-. In these cases, the electronic structure, insulating for the stoichiometric surface, is preserved upon adsorption. When the initial system does not favor an energy gap (open-shell adsorbates, defective surfaces), the best adsorption mode implies a redox mechanism and restores the situation of an insulator. In the case of open-shell adsorbates, an electron transfer to or from the oxide is a possible solution. The electron transfer occurs from the Substrate to the adsorbate for an electronegative group (Cl adsorption on Mn+) or the other way round for an electropositive one (NO adsorption on O). The reactivity at surfaces deviating from stoichiometric differs from that on the perfect ones, since the electron count is different. We will illustrate the previous concepts by the study of these adsorption processes on MgO and TiO2 surfaces. (C) 2003 Elsevier B.V. All rights reserved.