The role of hydrogen in carbon deposition on Ni has been studied at H-2/CO < 1 and 698 K by determining the respective rates of the carbon-forming reactions : (1) CO + H-2 --> C + H2O and (2) 2CO --> C + CO2. The steady-state rate of reaction (1) increases in proportion to H-2 pressure. On the other hand, reaction (2) is facilitated by the addition of an extremely small amount of H-2, so that the rate becomes about eight times that for pure CO but hardly varies as more H-2 is added. Similarly, there is a great difference in catalytic activity for ethylene hydrogenation between spent catalysts obtained in the deposition with and without H-2. These findings suggest that hydrogen, even in a small amount, makes free Ni surface area larger than for pure CO.