Combination of pyrolysis and expansion to a supersonic molecular beam is shown to be very effective in conversion of pure methane to heavier hydrocarbons. The conversion reaches similar to 70% in the supersonic nozzle made of quartz with the diameter of orifice equal 100 mu. The major products are acetylene, benzene and its homologues. Free radicals are also detected in the product distribution. The mechanism consists of the surface generation of the radicals, C-C bond formation, desorption to the gas phase, and expansion to the supersonic beam. It is suggested that relatively short gas-surface contact time (about 50 ms at T-oxide = 1000 degreesC) enables simultaneous kinetic and thermodynamic control.