Temperature-programmed desorption (He-TPD) and temperature-programmed reaction with hydrogen (H-2-TPR), carbon monoxide (CO-TPR) or methane (CH4-TPR) were carried out to elucidate the benzene formation mechanism as well as the role of CO during CH4-CO reaction over SiO2-supported Rh catalysts. The steady-state surface for the CH4-CO reaction was different from that of the CH4 decomposition reaction. The existence of benzene-like adsorbed species as "building blocks" was demonstrated on the CH4-CO reaction surface, while no such higher hydrocarbon adsorbed species was detected in the case of the CH4 decomposition surface. On the contrary, in CO-TPR experiments various unsaturated hydrocarbons were released from the steady-state CH4 decomposition surface, which was not the case from the CH4-CO reaction surface. It is concluded that adsorbed CO may play an important role to enhance the C-C bond formation of carbonaceous species, which correlates deeply with the novel phenomenon of selective benzene formation in the CH4-CO reaction.