Abstract This paper describes the synthesis and discusses the thermal stability, oxygen and cation stoichiometry, and electrical transport properties of Cd/Mn spinel materials. It has been found that an effective synthesis can be obtained by reacting in the solid state CdO and Mn 2O 3 in a closed quartz ampoule at 800°C. By following this synthetic procedure it was possible to prepare materials with different Cd/Mn ratios. The as-prepared spinels are slightly understoichiometric in the oxygen content, but at appropriate T and P(O 2) they became overstoichiometric. The electrical transport takes place by motion of electron holes that are created by oxygen overstoichiometry at low T. At higher temperatures a different conduction mechanism is active. This mechanism is likely related to the thermal promotion of electrons from the valence into the conduction band.