Lanthanum erbium carbide, La 0.5-Er 0.5C 2, a salt-like carbide with a cubic fluorite phase structure, has been produced from 13C, allowing carbon diffusion rate to be determined using 12C. Carbon in salt-like carbides exhibits significant ionicity, and a high carbon diffusion rate would enable a new class of high temperature fuel cells based on carbon-ion transport. The complete lack of carbon diffusion data for salt-like carbides is the motivation for this work. The carbon diffusion rate in La 0.5Er 0.5C 2 has now been determined to be 2.0±0.8×10 -13 cm 2/s at 850 °C, increasing to 1.8 ±0.8×10 -11 cm 2/s at 1150 °C, with an activation energy of about 95 kJ/mole. These diffusion rates are too low for a carbon-ion fuel cell, but a number of other salt-like carbides exist. Be 2C, in particular, is a salt-like carbide with an antifluorite structure, and should have higher carbon-ion diffusion than cubic La 0.5Er 0.5C 2 due to the unoccupied octahedral sites in the antifluorite structure, but Be 2C presents special difficulties due to the toxic nature of its hydrolysis products. © ASM International.