It has long been believed as a law that electrons and mobile ions migrate independently in a mixed ionic electronic conductor compound. We have experimentally found that it is only an exception: the interference effect between electrons and ions upon their transfer is by no means negligible depending on the thermodynamic state of the system. We have also experimentally verified, with the highest ever precision, the Onsager reciprocity in the phenomenon of mixed conduction in solid state. (C) 2008 Elsevier B.V. All fights reserved.