The stability of (Ln(0.8)Ca(0.1)Ln'(0.1))(2)Ti2O7-delta(Ln=Dy, Yb; Ln'=Ce, Tb) pyrochlores under different redox conditions (air, dry and wet H-2) has been studied. The bulk conductivity of the terbium-containing materials is slightly higher under reducing conditions (5% H-2 + 95% Ar) than in air, reaching similar to 6 x 10(-2) and 2.5 x 10(-2) S/cm at 800 degrees C for (Yb0.8Ca0.1Tb0.1)(2)Ti2O7-delta and (Dy0.8Ca0.1Tb0.1)(2)Ti2O7-delta, respectively. In air-hydrogen-air cycles, the bulk conductivity returns to its original level in air: similar to 2 x 10(-2) and 1 x 10(-2) S/cm at 800 degrees C in (Yb0.8Ca0.1Tb0.1)(2)Ti2O7-delta and (Dy0.8Ca0.1Tb0.1)(2)Ti2O7-delta respectively. Exposure of (Dy0.8Ca0.1Tb0.1)(2)Ti2O7-delta to a flowing mixture of 5% H-2 and 95% Ar for 100 h slightly reduces its conductivity: from 0.029 to 0.023 S/cm at 900 degrees C. The cerium-containing materials (Yb0.8Ca0.1Tb0.1)(2)Ti2O7-delta and (Dy0.8Ca0.1Ce0.1)(2)Ti2O7-delta were found to be unstable under reducing conditions. The bulk conductivity of (Yb0.8Ca0.1Ce0.1)(2)Ti2O7-delta is practically independent on the atmosphere while the grain boundary conductivity increased from 5.6 x 10(-8) in air to similar to 1 x 10(-5) S/cm at 425 degrees C on reducing the sample in 5%H-2-Ar gas mixture. However, after two redox cycles air-5%H-2-Ar-air the sintered pellet of (Yb0.8Ca0.1Ce0.1)(2)Ti2O7-delta fractured whereas (Dy0.8Ca0.1Ce0.1)(2)Ti2O7-delta degraded immediately under reducing conditions. It was observed that the size mismatch between Ln and Ln cations strongly affects the bulk to grain boundary conductivity ratio in (Ln(0.8)Ca(0.1)Ln'(0.1))(2)Ti2O7-delta (Ln=Dy, Yb; Ln'=Ce, Tb) pyrochlores. (C) 2012 Elsevier B.V. All rights reserved.