New phase diagram data and a thermodynamic assessment of the CeO-Gd2O3-CoO system using the CALPHAD approach are presented. This information is needed to understand the surprisingly low sintering temperature (950 degrees C1050 degrees C) of CeO2-based materials doped with small amounts of transition metal oxide (e.g., CoO). Experimental phase equilibria between 1100 degrees C and 1300 degrees C are reported based on the analysis of annealed and molten samples. No isolated compound exists in the ternary. At 1300 degrees C the Co solubility in the ternary compounds Ce1xyGdxCoyO2x/2y (fluorite) is 2.7mol% and is less than 1mol% in the Gd2xCexO3+x/2 (bixbyite). The Ce solubility in the perovskite GdCoO3 was found to be 1mol%. The lowest temperature eutectic melt in the ternary has a composition of 57.2mol% Co and 41.1mol% Gd melting at an onset temperature of 1303 +/- 5 degrees C, which is close to the binary eutectic in the Gd2O3-CoO system at 60 +/- 2mol% Co and 1348 +/- 1 degrees C.