The oxidative methylation of acetonitrile to acrylonitrile with methane for temperatures in the range 550-730 degrees C over Li/ MgO follows a radical mechanism. The reaction proceeds via the formation of radicals at the cr-carbon of acetonitrile and methyl radicals from methane. The coupling of these radicals leads Co propionitrile which is further transformed to acrylonitrile via oxidative dehydrogenation. Experimental evidences indicate that the reaction is Langmuir-Hinselwood. The Li+O- surface sites of Li/MgO are the active centers for the activation of both methane and acetonitrile. Oxygen is absolutely necessary for the formation of the corresponding radicals from methane and acetonitrile but it must be provided at a controllable manner in order to avoid undesired oxidation reaction of nitriles. The decomposition of acetonitrile which would lead to CH4 and HCN does not take place. However, the increase of the nitrile chain length favors the breaking of the C-C bond between the cyanide group and the alpha-carbon of the corresponding nitrile.