The thermoelectric power and electrical conductivity of minerals may provide valuable information on microscopic properties such as intersite cation distributions and defect structure if the mechanism by which electrical conduction takes place could be determined. Theoretical considerations and experimental measurements point convincingly towards an electron hopping (small polaron) conduction mechanism for several transition metal oxides, including magnetite and wüstite, as well as solid solutions involving those end-members. It can furthermore be shown that conduction takes place on octahedral sites only. In the case of magnetite and solid solutions involving MgFe₂O₄, FeAl₂O₄, FeCr₂O₄, and Fe₂TiO₄, we demonstrate how in-situ, high temperature measurements of thermoelectric power and electrical Conductivity can be used to calculate the distribution of Fe²⁺ and Fe³⁺ between octahedral and tetrahedral sites. It is more problematic to interpret the electrical poperies of wüstite and solid solutions involving MgO in terms of the microscopic defect structure because of cluster formation. We have found, however, that for a solid solution containing 83% MgO, the thermoelectric power is essentially consistent with the presence of Fe³⁺ - V″_Fe – Fe³⁺ clusters as originally suggested by Brynestad and Flood (1958).