The properties of directionally solidified eutectic (DSE) zirconia-mullite composites are presented. These materials combine two oxide ion conductors in eutectic microstructures stable over broad temperature (570-1415 degrees C) and PO2 (10(-20) to 10(5) Pa) ranges, which are pertinent for their application as high temperature (>1200 degrees C) solid electrolytes, e.g. in Nernstian sensors. X-ray diffraction and scanning electron microscopy/energy dispersive spectroscopy results reveal a composite structure comprising eutectic crystals of mullite and zirconia, and an intergranular amorphous phase rich in Al-Y-Si-O. The amorphous phase is crystallised upon annealing at 1400 degrees C, and the resulting composites have a composition close to the nominal eutectic consisting of 79 vol.% mullite and 21 vol.% zirconia. The electrical conductivity of these materials is rationalized in terms of percolation, fraction and properties of each phase, attaining values in excess of 0.01 S/cm at 1370 degrees C and displaying Arrhenius behaviour with activation energy of 70 kJ/mol. The broad electrolytic domain of these solid electrolytes is demonstrated by conductivity measurements carried out from 600 degrees C to 1370 degrees C in O2, air, Ar and 10%H-2 + 90%N-2 atmospheres. (C) 2013 Elsevier B.V. All rights reserved.