A rigorous kinetic model for the formation and gasification of filamentous carbon on a nickel steam-reforming catalyst is derived. Carbon formation and gasification experiments were performed in an electrobalance unit at temperatures ranging from 773 to 823 K and partial pressures of the various components in the range encountered in industrial steam reformers. The rates of growth or gasification of the carbon filaments were always based on the same number of carbon filaments. The gasification by carbon dioxide and by steam proceed through adsorbed oxygen, generated by the dissociation of steam and carbon dioxide. Relationships between the various carbon formation and gasification reactions are accounted for, to ensure that the kinetic model is thermodynamically sound. The lower net rate of carbon formation on an alkalized catalyst can be explained by a lower lumped forward rate coefficient for the methane cracking and an increased surface concentration of oxygen.