Non-metallic elements such as carbon, oxygen, and nitrogen in liquid sodium exert a significant effect on the material behaviour of cell construction materials in liquid metal battery systems. Austenitic Fe-18% Cr-8%Ni foil and alpha-iron foil are equilibrated at 550 degrees C in sodium, and, subsequently, the concentration of carbon in the foils is analysed. A new relationship has been obtained between carbon activity and carbon concentration in alpha-iron foil, and is compared with the previously developed ones involving 304 SS foil reference samples. An equilibrium method has been developed to measure the activity of carbon in liquid sodium by using alpha-iron with this new expression. The results show similar values to those obtained with the 304 SS foil reference samples. (C) 2015 Elsevier B.V. All rights reserved.