In order to identify new oxide ion-conducting materials in the ceria family of oxides, the unique effect of co-doping is explored and a novel series of Ce0.8Sm0.2-xCaxO2-delta Compositions is identified that have enhanced properties compared to the singledoped Ce0.8Sm0.2O1.9 and Ce0.8Ca0.2O1.9 cornpositions. Moreover, the superior characteristics of the co-doped Ce0.8Sm0.2-xCaxO2-delta powders prepared by the mixed-fuel process aid in obtaining 98 % dense ceramics upon sintering at 1.200 degrees C for 6 h. Though a linear increase in conductivity is observed by replacing Sm with Ca, the composition with the maximum amount of Ca and the minimum amount of Sm exhibits a significant improvement in properties compared to the rest in the series. The composition Ce0.80Sm0.05Ca0.15O2-delta exhibits a conductivity as high as 1.22 x 10(-1) S cm-l at 700 degrees C with minimum activation energy (0.56 eV) and a superior chemical stability to reduction compared to any of the hitherto known (CaSm) compositions. The absence of Ce-III, confirmed both from X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, strongly suggests that the observed increase in conductivity is solely due to the oxide ion conductivity and not due to the partial electronic contribution arising from the presence of Ce-III and Ce-IV. To conclude, the experimental results on the Ce0.8Sm0.2-xCaxO2-delta series underscore the unique effect of calcium co-doping in identifying a cost-effective new composition, with a remarkably high conductivity and enhanced chemical stability to reduction, for technological applications.