The role of a molybdenum addition to Rh/Al2O3 catalysts has been studied in the decomposition of methane and subsequent hydrogenation reactions of the hydrocarbonaceous residues produced. Mo existed as a dispersed phase over the alumina support, which, following reduction, was present as exposed isolated Mo2+ ions and as a surface layer on Rh particles which reduced the latter's propensity to adsorb CO. Methane underwent decomposition at 673 K to generate CxH and other hydrocarbonaceous species which blocked surface sites on Rh/Mo/Al2O3 in the order bridging sites on Rh, Rh gem-dicarbonyl, Mo2+, and Rh on-top sites. A further proportion of the coke generated was located on the alumina support surface. Subsequent hydrogenation of the coked Rh/Mo/Al2O3 catalyst showed two maxima in product formation, the first between room temperature and 350 K and the second between 375 and 475 K, whereas methane was detected only at ca. 473 K for Rh/Al2O3 in the absence of Mo. The presence of Mo in the Rh/Al2O3 catalyst enhances the extent to which coke removal takes place in hydrogen at low temperatures, possibly by restricting the mobility, and thus the ageing, of the residues. Restructuring of the carbonaceous residues occurs in the presence of CO for Rh/Al2O3 but is less significant for Rh/Mo/Al2O3.