Ca-doped (1; 1.7; 5 and 10mol% CaO) cobalt oxide single crystal samples, having an [100] orientation, were annealed at 1100degreesC in air or argon atmosphere. The microstructure was examined by means of transmission electron microscopy (TEM). The electron energy loss spectroscopy (EELS) analysis of the matrix and precipitates has been performed in order to understand the conductivity behavior of these materials. An extensive precipitation of Ca-free Co3O4 spinel crystals was observed with increasing Ca content and oxygen activity. It is suggested that the electrical conductivity changes in this material may be related to this precipitation, because it changes the electronic state of cobalt cations. (C) 2003 Published by Elsevier Science B.V.