The ionic conductivity of Yb2Ti2O7-based materials can be improved by compositional changes, including deviations from the nominal cation ratio Yb:Ti, combined with optimization of high temperature processing. Structural refinement indicates that a slight spontaneous anti-site occupancy occurs in stoichiometric Yb2Ti2O7, and that major anti-site occupancy occurs in compositions with excess of Yb and excess of Ti. Yb-excess should thus act as an acceptor-type additive, with increase in ionic transport, suppression of the electronic contribution, and an extended electrolytic domain. However, the actual results show that a small Yb-excess actually suppresses the ionic conductivity. On the contrary, the highest conductivity in air was found for samples with slight excess of Ti in spite of its expected donor-type character. The increase in electronic conductivity of Ti-excess compositions is also weaker than expected. These effects can be explained by significant structural differences between off-stoichiometric and stoichiometric compositions. Furthermore, the actual results also suggest that the nominal composition Yb2Ti2O7 is likely to undergo structural changes on cooling from sintering temperatures, and thus dependence of transport properties on thermal history. (c) 2008 Elsevier B.V. All rights reserved.