The mass action law formalism has been extensively used as an efficient way to rationalize a large spectrum of properties of non-stoichiometric oxides. This approach is still extremely useful in spite of some shortcomings. In fact most of the improvements proposed in the literature have lost the clarity of the initial model and convey an extremely heavy formalism. We show that an obvious improvement is to take into account the electrostatic interactions, which allows to better understand not only thermodynamic properties but also transport properties.