The effect of varying co-doping ratios on the grain and grain boundary conductivity is evaluated in the Sm x Nd 0.15-x Ce 0.85 O 2-δ system. A slight decline in the grain conductivity is found on replacing Sm 3+ with Nd 3+ in Sm x Nd 0.15-x Ce 0.85 O 2-δ . Nevertheless, the activation energy values for grain conductivity are comparable for all the compositions. The specific grain boundary conductivity calculated at lower temperatures, is found to be nearly 3 orders of magnitude smaller than the grain conductivity. Further, a wide variation in specific grain boundary conductivity data is observed across various tested compositions, with Sm 0.10 Nd 0.05 Ce 0.85 O 2-δ exhibiting the highest conductivity. The high specific grain boundary conductivity contributes towards high total conductivity in this composition.