The steady-state rates of simultaneous carbon formation and methanation on silica-supported nickel and nickel-copper catalysts in CO + H-2 gas mixtures have been measured in the partial pressure range 10-30 kPa and the temperature range 583-873 K. The steady-state kinetic results for carbon formation can be explained by a model previously used for carbon formation in pure CO gas, but modified to take into account the influence of hydrogen. Comparisons between the results for Ni/SiO2, Ni0.99Cu0.01/SiO2, Ni0.9Cu0.1/SiO2, and Ni0.75Cu0.25/SiO2 show that the specific rate of carbon formation is increased when 1 at.% of Cu is added to a nickel catalyst, but that the rate decreases when the Cu content is increased to 10 at.%, and that no carbon formation is seen for the Ni0.75CU0.25/SiO2 catalyst. The addition of a small amount of Cu, on the other hand, decreases the methanation rate while at higher Cu contents the rate goes up again and a maximum is observed at about 7 at.% Cu. Thus a simple ensemble model cannot explain the influence of copper on the rate of carbon formation or methanation. Comparison of carbon formation and methanation rates for the Ni/SiO2 and the Ni0.99Cu0.01/SiO2 catalysts indicates that the two reactions have different rate-controlling steps and that the carbon formation influences the methanation rate mainly through poisoning of the catalysts.