It is a well‐known result of semiconductor physics, that the conductivity of a semiconducting oxide can be shifted in the direction of increasedpconductivity or decreasednconductivity by addition of small proportions of oxides of lower cation valency and that the reverse is true in the case of higher valency of the added cation. This effect is understood on the basis of the corpuscular semiconductor theory of Wagner and Schottky and is even used for establishing the nature of the conduction mechanism in certain materials. It is generally supposed that the only necessary condition for this doping effect is solubility in the solid state and that this is granted by a similarity of the cation radii. No importance has been attributed to the chemical nature of the dope, and for this reason we have tried to dopep‐conducting nickel oxide with other oxides, differing not only in valence but also in their chemical nature.