The effect of Co dispersion, reaction temperature and the presence of CO or H-2 on the kinetics of CH4 decomposition over 5-30 wt.% CO/SiO2 catalysts is reported. The initial CH4 decomposition activity increased and the catalyst deactivation rate decreased as Co dispersion decreased in the range 13-5%. The deactivation rate increased with reaction temperature because of rapid ageing of the deposited carbon. Indirect evidence for CHx migration from the metal to the support is presented. The migration is much less important than filamentous carbon formation in removing carbon from the metal surface and thereby maintaining catalyst activity during CH4 decomposition. The presence of H-2 or CO reduced the net rate of carbon deposition and increased the net rate of carbon removal by diffusion through the Co, respectively. Thus, under the appropriate reaction conditions and in the presence of H-2 or CO, stable CH4 decomposition activity and filamentous carbon formation were observed on supported Co catalysts with low metal loading.