A kinetic model of CH4 decomposition on supported Co catalysts, that describes the stable catalyst activity associated with filamentous carbon formation and the catalyst deactivation associated with the formation of encapsulating carbon, is presented. The rate of carbon nucleation at the tailing face of the supported metal particle, a precursor for the growth of filamentous carbon, is described by a cluster nucleation model. The deactivation or stable activity, observed after an initial rate increase that is associated with carbon nucleation, is explained by the competition between the rate of carbon diffusion through the metal particle and filamentous carbon growth, versus the rate of encapsulating carbon formation on the leading face of the particle. The site density profiles through the metal particle at different reaction times are also calculated, and the effect of metal particle size on the initial CH4 decomposition rate is quantified. (c) 2005 Elsevier Inc. All rights reserved.