Activation of methane is carried out by means of an air microwave plasma (2.45 GHz). The experiments cover the absorbed microwave power range 350-650 W (20-50 W cm(-3)) with 17-62% of methane in the gas mixture, with pressures of 10-66 mbar and flow rates of 140-700 ml min(-1). Methane, dioxygen, and dinitrogen consumptions as well as C-2 hydrocarbons, carbon monoxide, and dihydrogen yields are analyzed by gas chromatography. The distance of methane addition from the end of the discharge plays an important role in the composition and the concentration of the products obtained. This distance mainly determines the energy concentrated in the active species of the plasma when they react with methane. A of C-2 hydrocarbons and carbon monoxide is discussed based on the experimental results and kinetic data in the literature.