Monophase cubic spinel-type oxides of Mn1.5Co(0.25+X)Ni(1.25-X)O4 (0 <= X <= 1) were prepared through the oxidation of specimens sintered at 1400 degrees C. The oxides with 0 <= X <= 0.75 composition are focused on in this study, as the oxides with 0.75 < X <= 1 composition did not convert into a monophase cubic spinel structure. The electrical conductivity of the sintered bodies was confirmed to increase exponentially with increasing temperature. In the composition range of 0 <= X <= 0.75, both the electrical conductivity and the Seebeck coefficient increased with increasing X. The oxides with composition between 0 <= X < 0.57 were n-type semiconductors, whereas those with 0.57 < X <= 0.75 were p-type. It was concluded that electrical conduction in the specimens is controlled by small polaron hopping.