Controlled atmosphere electron microscopy observations of the nickel-catalyzed growth and gasification of carbon filaments have shown that these processes can be reversed. This supports the view that growth and gasification in either hydrogen or steam occur by similar mechanisms, where one of the steps involves the diffusion of carbon through the metal. It was observed that the small catalyst particles are the most active in filament formation and steam gasification, while the large particles are the most active for the hydrogenation reaction. This is explained in terms of different rate-controlling steps.