The adsorptive properties of two catalysts for methane coupling, namely 2% Li2O-Y2O3 and LiYO2, have been studied by means of temperature-programmed desorption (TPD) in order to gain a deeper insight into the reaction mechanism and to distinguish the role of the heterogeneous surface reaction and homogeneous gas phase reaction. Experiments concerning the temperature-programmed desorption of methane and oxygen (adsorbed either at room temperature or at 973 K) have been therefore carried out. In order to improve their characterization, TPD peaks have been also analyzed from a kinetic point of view. Afterwards, carbon dioxide desorption has also been evaluated, its presence on the catalyst surface being considered decisive in determining the prevailing reaction mechanism. The possibility of detecting, by means of a mass analyzer, that the desorption process of methane is accompanied by its surface oxidation has been decisive in the characterization of its interactions with the catalysts surfaces. The results seem to indicate that carbonate surface species are important for the adsorption of methane, but the formation of C-2-hydrocarbons mainly occurs in the gas phase.