A model of the kinetics of CH4 decomposition on supported Co catalysts, at conditions relevant to the two-step CH4 homologation reaction (450 degrees C, 101 kPa), is presented. The model includes the decomposition of gas phase CH4 on a Co site and migration of the resulting CHx fragment from the Co to the support as the key kinetic steps in the decomposition reaction. The migration step is essential to explain the observed kinetics at long reaction times (2-7 min) or high CH4 feed concentrations (5% CH4 in Ar). At these conditions, CH4 decomposition continues at a low but constant rate despite a nominal Co coverage by CHx > 1. The magnitudes of the estimated rate constants for the decomposition and migration steps are used to interpret the observed effects of temperature, catalyst metal loading, support, and K promoter on the CH4 decomposition kinetics.