In this work we studied the electronic conductivity of highly reduced polycrystalline Ce0.8Gd0.2O1.9-delta at low temperatures (25 - 100 degrees C). The specimens were treated at a variety of reducing conditions at high temperature and then quenched. The nonstoichiometry of the samples was estimated by thermogravimetry. We showed that a high reduction level leads to high conductivity and low activation energy. We estimated an activation energy change from 0.60 to 0.38 eV in going from slightly to heavily reduced. An X-ray diffraction study of this nonstoichiometric material showed that the reduction is accompanied by an increase of a secondary bixbyite phase. If the quenched structures studied in this work represent the high-temperature phases, then our results are in contradiction to models that consider that the fluorite structure of CeO2 is highly tolerant to reduction remaining as a single phase. (c) 2006 The Electrochemical Society.