In the thermodynamic analysis of pyrometallurgical processes using coke as a reductant, the data for CO and CO[sub 2] from thermochemical tables, relative to graphite as the standard state for carbon, are used. Since coke is an amorphous form of carbon, its Gibbs energy is expected to be higher than that of graphite. However, there has been no direct measurement of the stability of coke relative to graphite reported in the literature. For an accurate description of the thermodynamics of reactions involving coke, it is necessary to obtain quantitative values for the Gibbs energy difference between coke and graphite as a function of temperature. This communication reports an experimental determination of the activity of carbon in coke relative to graphite using a solid-state galvanic cell incorporating single-crystal CaF[sub 2] as the solid electrolyte. The results obtained in this study indicate a significantly higher solubility of coke compared to graphite in iron. However, metals are known to catalyze graphitization and the surface of coke in contact with the metal is likely to have a graphitelike structure. Thus, the theoretical difference in solubility may not be measurable in practice.