The electromotive force (e.m.f.) of a solid oxide fuel cell (SOFC) based on fluorite structure electrolyte is calculated by defect chemistry analysis. This is strongly influenced by the characteristic oxygen pressure, P-O*, at which the ionic transference number of the electrolyte is 0.5. It is concluded that e.m.f. decreases with increasing temperature to that of a purely ionic electrolyte-based fuel cell. The low anodic oxygen partial pressure has a smaller influence on the SOFC based on an electrolyte with mixed conduction than on the one based on a purely ionic electrolyte.