The reaction between carbon and oxygen in molten iron is one of the most fundamental reactions in the iron- and steelmaking process, and many experiments have been carried out and the results are well summarized by Kawai and Mori.1)However, the close investigation on the interaction parameters shows very interesting factor. The values of interaction parameter between carbon and oxygen in weight percent standard state, eO C, determined in the earlier studies2–6) are in range from 0.4 to 0.6, while the values from relatively later studies7–9) are in range from 0.3 to even 0.1. The interaction parameter between carbon and oxygen eO C was mostly determined from the equilibration between CO/CO2gas mixture and molten Fe–C metal contained in CaO, MgO, Al2O3or graphite crucible. In certain cases, a reaction furnace was designed to carry out the equilibration under high pressure up to 80atm. However, several difficulties of the experiments have been pointed out: (i) precise control and analysis of CO/CO2 ratio in gas due to very low CO2 level, (ii) evolution of CO gas during the solidification of samples after equilibration and consequent addition of Al to prevent this oxygen loss, (iii) possible contamination of oxygen due to the adsorption of oxygen to graphitic specimens, (iv) very sluggish reaction between Fe–C melt and CO/CO2gas especially at high C concentration. In the present study, several representative experimental studies will be critically reviewed again, and the thermodynamics of the Fe–C–O system will be reassessed based on more recent thermodynamic description of the Fe–C system.