–Abstract: A device of the form Metal1|MIEC|Metal2 is discussed, MIEC being a mixed ionic electronic conductor which is assumed to include one mobile ionic defect (acceptors) and one electronic defect (holes). The two electrodes are assumed to be inert and blocking for material exchange. The defect distribution and I–V relations are solved numerically assuming steady-state and local equilibrium. The effects of acceptor ionization, contact potentials and space charge are taken into consideration. Different thicknesses, from nano to macro, are considered as well as different contact potentials. The results are compared with the local neutrality (analytic) limit. It is found that the device when based on thin MIEC and certain a-symmetric contact potentials shows rectification somewhat similar to a classic Schottky diode, implying that aging process may not be a real problem. A second model is considered in which the ionic motion is replaced by electron hopping between acceptors. The model is solved numerically for the defect distributions and I–V relations. This and the previous models are compared for different thicknesses and contact potentials. The two models may present one of four families of I–V curves (though each model may exhibit them under different conditions). Thus the hopping process cannot be distinguished from the one with ionic motion by just examining the I–V relations and further information is required for identifying the model.