Abstract Negative temperature coefficient thermistors have been prepared with nickel-copper-manganite spinels, Mn7/3-x-Ni2/3CuxO4. These devices are very sensitive, but unstable with time and storage temperature. The addition of Ba to these ceramics decreases this instability from 15% to less than 2%. To study this change in the electrical properties of the ceramics we used R(T), I(V) and thermopower measurements. The correlation of ionic distribution and the results obtained from Seebeck measurements allowed us to propound the transfer of Cu2+ between tetrahedral and octahedral sites to explain this phenomenon. We also assume that the Cu2+ ions in the octahedral site contribute to the electrical conductivity.