An equilibration method has been developed to measure accurately the activity of carbon in liquid sodium. Foils of Fe - 8 wt% Ni, Fe - 16 wt% Ni, and Fe - 18 wt% Cr - 8 wt% Ni alloys can be exposed to liquid sodium at temperatures be tween 650 and 800°C for times necessary to achieve 99% of the equilibrium carbon concentra tion in the alloys. The carbon activity-concentra tion relationships for these materials, based on the graphite standard state, were determined from equilibration experiments in CH4-H2 gas mix tures. Carbon activity measurements in flowing sodium at 750°C by the foil equilibration method were used to calibrate the response of a diffusion-type carbon meter that was useful in monitoring the carbon activity in sodium. No correlation was obtained between measured carbon activities by the foil equilibration method, in the low activity range of interest in reactor sodium systems, and either carbon analyses of sodium samples or the response of an electrochemical carbon meter.