Understanding the effect of stress on the properties of oxides used in a solid oxide fuel cell (SOFC) is crucial for predicting its performance. The stress can come from many sources, such as compositional strain or thermal expansion mismatch between various constituents of the cell. These two effects can combine and alter the electronic structure as well as the defect chemistry of the oxide. In this paper, the effects of stress on the electron hole and oxygen vacancy concentrations and conductivities are addressed in mixed ionic electronic conductors (MIECs) in equilibrium with oxygen. Furthermore, two types of mixed conducting oxides are examined, and treated with the same formalism to illuminate the effects of stress on these materials. Applications of this model include SOFCs and thin films.