Bulk and grain boundary conductivities of Yb2+xTi2-xO7-x/2 (x = 0, 0.1, 0.18 and 0.29) materials were studied by impedance spectroscopy in the range 300-900 degrees C in air. Ionic and electronic conductivities were separated by both ion blocking Hebb-Wagner measurements and total conductivity measurements as a function of oxygen partial pressure in the temperature range 700-1000 degrees C. The oxygen partial pressure dependence of the total conductivity shows that these materials are nearly pure ionic conductors in air and that the ionic conductivity decreases for Yb-rich compositions. This was interpreted as a predominant effect of a decrease in mobility of ionic charge carriers, opposing the expected increase in concentration of oxygen vacancies with increasing Yb content. The studied materials become mixed conductors under typical fuel conditions, except possibly at temperatures below about 700 degrees C. Yb-excess slightly suppresses the electronic conductivity. (c) 2006 Elsevier B.V. All rights reserved.