The promotion of Co catalysts with K has been examined for the two-step CH4 homologation reaction. The effect of K was strongly influenced by the catalyst support. In the case of the SiO2 supported catalyst, addition of K increased the CH4 decomposition activity and decreased the second-stage hydrogenation activity while the C-2+ selectivity increased from 14 C at% to 36 C at%. With the Al2O3 support, addition of K increased CH4 decomposition activity but the C-2+ selectivity increased only marginally. These results are discussed in terms of Co dispersion, support effects and the effect of K on the reactivity of the carbon species deposited during CH4 decomposition.