A co-dopant strategy is used to investigate the effect that the elastic strain in the lattice has on the grain ionic conductivity of doped ceria electrolytes. Based on critical dopant ionic radius (r(c)) different compositions in the LuxNdyCe1-x-yO2-delta (x+y=0.05, 0.10, 0.15, and 0.20) system are studied. Dopants are added such that the weighted average dopant ionic radius matches r(c) for all the compositions. Dense ceramic discs are prepared using conventional solid oxide route and sintering methods. Precise lattice parameter measurements are used to calculate the lattice strain. The ionic conductivity of the samples is measured in the temperature range of 250 degrees C to 700 degrees C using two-probe electrochemical impedance spectroscopy technique. The elastic strain present in LuxNdyCe1-x-yO2-delta system is found to be negligible when compared to LuxCe1-xO2-delta (negative) and NdxCe1-xO2-delta (Positive) systems. Grain ionic conductivity of LuxNdyCe1-x-yO2-delta (where x+y=0.05) at 500 degrees C is observed to be 1.9 X 10(-3) S/cm which is twice as high as that Of Lu0.05Ce0.95O2-delta. These results extend the validity of the r(c) concept as a strategy for co-doping ceria electrolytes and open new designing avenues for solid oxide electrolytes with enhanced ionic conductivity. (c) 2006 Elsevier B.V. All rights reserved.