Mixed Ionic and Electronic Conductors (MIECs) are promising, membrane materials for oxygen separation from air at high temperature. The transient stage is critical due to the stress induced by the chemical strain, combined with the oxygen activity field through the membrane. In the literature, permeation models are able to predict the oxygen flux in steady state or to identify the slowest mechanism. In this paper, the proposed model aims to predict the oxygen activity fields through the membrane both in transient stage and steady state. A macroscopic approach is Proposed, the balance of a temporary species is introduced to take into account separately the dissociative adsorption, associative desorption and interface reaction between surface and bulk. New densities of probability are proposed to model dissociative adsorption and associative desorption in order to take into account the difference between "classical" catalysis and the MIECs case. This model is implementing in the finite element software COMSOL Multiphysics thanks to the combination of the modules "coefficient form PDE" and "coefficient form boundary PDE". The results of the model are in good agreement with steady state and transient stage experimental data. The different kinetics of oxygen release and oxygen intake are well reproduced.