A mathematical model describing the distribution of the concentrations of electrons and electron holes and the potential along the thickness of electrolyte is developed. This model facilitates the study of various electrolyte materials and the effects of operating temperature, oxygen partial pressure and electrolyte thickness on oxygen semi-permeability. The model is applied to a study of the performance of a ceramic oxygen generators (CoGs) and a solid oxide fuel cells (SoFCs). In a CoG, CO + CO(2)lelectroloytelpure O-2 and N-2 + O(2)lelectroloytelpure O-2 are considered for a system which requires low O-2 and high O-2 semi-permeability, respectively. In a SoFC, for doped-ZrO2 at 800 degreesC, the thinner the electrolyte, the higher the energy efficiency and the output power density will be at intermediate current density due to the higher Ohmic loss of the electrolyte. Thus, obtaining low O-2 semi-permeability for a thin electrolyte is also desirable in SoFC development.