A diffusion-cell type of carbon meter is useful for continuous measurement of the carburizing potential of liquid sodium. The iron sensing element is small and long lived and responds rapidly to changes in carburizing potential. Measured carbon fluxes through the iron probe wall ranged from 0.007 to 1.7 µg/(cm2 min) in response to carburizing additives to the sodium. Stainless steel tabs in the sodium carburized at a rate that varied from negligible to rapid over this range. Carbon monoxide and unstable carbides were found to produce a high carbon activity in the sodium, whereas elemental carbon in the absence of oxygen had little effect.