The purpose of this investigation was to determine the primary reaction responsible for carbon deposition when Fe and Co foils were exposed to a five-gas mixture of Hz, CO, CH,, CO,, and Hz0 at 900 K and 1 atm. Binary (CO--CO*, and CHh-H2) gas mixtures were used to study the relative rates of carbon deposition; however, the role of H, as a facilitator with CO-CO2 gas mixtures is left unanswered. A ternary gas mixture of Hz--CO-H,0 can be used to investigate the role of H,; however, this gas mixture results in an infinite 2, for carbon deposition via the Boudouard reaction. In order to s~multaneousIy control the gas-phase carbon activity of both the Boudouard reaction and the hydrogenation of CO, it is necessary to use a four-gas mixture of Hz, CO, CO,, and H,O. Reactions were carried out over Fe and Co foils at 900 K and 1 atm. The rate of carbon deposition was determined as a function of gas phase carbon activity (ir<) of the Boudouard reaction and the hydrogenation of CO. By carefully choosing different four-gas mixtures, it is possible to simultaneously adjust the ii, of each reaction such that the a, of CO hydrogenation is increasing, while the activity to form carbon deposits via the Boudouard reaction is decreasing, and vice versa. In this manner, the relative driving force for carbon deposition from each reaction can be controlled with Hz still present in the gas mixture, thus eliminating the question as to whether HZ enhances the Boudouard reaction. Results indicate that the hydrogenation of CO is the main reaction for carbon deposition at 900 K and 1 atm. Also, CO hydrogenation appears to govern carbide formation.