The movement of ions and electrons in a solid or liquid phase is characterized in the first place by their mobility, i.e. the steady-state drift velocity in cm/sec with respect to an appropriate reference system at unit strength of the electrical field (1 volt/cm). Further information is obtained from the temperature dependence of the mobility which may be used in order to calculate the activation energy with the help of an Arrhenius plot. Moreover, thermoelectric measurements may be evaluated in order to obtain heats of transfer of mobile species in accord with the definition due to Eastman31) Previous authors have considered mostly the emf of thermocells involving either metals or semiconductors with prevailing electronic conduction, or thermocells involving either aqueous solutions or ionic crystals exhibiting ionic conduction. This article is concerned with the evaluation of the emf of thermocells which involve ionic crystals or melts exhibiting both ionic and electronic conduction. The evaluation of the Soret effect and measurements of the current of short-circuited thermocells may yield auxiliary information. Consider a thermocell involving a solid or liquid compound of metal A and nonmetal X with the absolute valences z A and Izxl, respectively. At the ideal stoichiometric composition the ratio of anions to cations is, therefore, v = ZA/IZxl corresponding to the formula AX,. To account for deviations from the ideal composition, the formula A 1 +,Xv is used where denotes the metal excess. In the case of a metal deficit, fi has a negative value. The occurrence of deviations from the ideal stoichiometric composition has the following consequences. First, a temperature gradient brings about thermal diffusion, i.e. enrichment of the metal excess at the hot or the cold end of a sample of compound AXe, also denoted as Soret effect. Second, the thermoelectric power of a cell involving compound A Xv depends in general, on the value of ~, or a related thermodynamic quantity, e.g. the activity of metal A.