The first results are presented on heterovalent substitution of alkaline-earth cations Ca2+ for Yb3+ in oxide ion conductor Yb2Ti2O7. (Yb1-xCax)(2)Ti2O7 (x = 0.05, 0.1) has been synthesized using mechanical activation of the oxides Yb2O3, CaO, TiO2 and subsequently sintering at 1400 degrees C in air, and their electrical conductivity has been measured from 300 to 950 degrees C. (Yb0.9Ca0.1)(2)Ti2O7 synthesized at 1400 degrees C from a mechanically activated oxide mixture has the highest conductivity among the pyrochlores studied so far: 2 x 10(-2) S/cm at 740 degrees C, i.e., 0.2 S/cm at 1000 degrees C. Neutron diffraction data indicate that the Ca2+ in (Yb0.9Ca0.1)(2)Ti2O7 occupies the ytterbium site, leading to the formation of oxygen vacancies in the O2 (48f) site and the presence of oxygens in the O3 (8b) site, normally vacant in ordered pyrochlores. No evidence of Ca substitution on the Ti site has been found. (c) 2008 Elsevier B.V. All rights reserved.