A detailed description is given of the formation and the gasification of filamentous carbon. The diffusion of carbon through nickel originates from a concentration gradient, which implies a different solubility at the nickel-gas and the nickel-carbon interface. A thermodynamic basis for the different solubilities is provided. The segregation of carbon, taking place at the gas side of the nickel particle, is added as one of the steps in the global mechanism of carbon filament formation and gasification. The segregation process may be described in a way similar to that of gas adsorption. The coupling of the surface reactions, the segregation process, and the diffusion of carbon through the nickel particle leads to a detailed model of the process of carbon filament formation, which forms the basis for the kinetic modeling of carbon formation and gasification reactions. Experimental results for the methane cracking revealed that the number of carbon filaments that is able to nucleate strongly depends upon the affinity for carbon formation.