Partial oxidative reactions of methane by carbon dioxide have been studied using atmospheric pressure alternating current plasmas. The reactions were carried out using a Y-type reactor with metal rods as the inner electrodes inside quartz tubes and aluminum foil wrapped around quartz tubes as the outer electrodes. The waveforms, input voltages, and currents of the reactions were monitored with an oscilloscope. Interactions between excited methane and excited carbon dioxide as well as those between one excited species and the other unexcited species were observed. The products of the reactions include carbon monoxide, hydrogen, ethane, ethylene, propane, and acetylene. The effects of many reaction parameters, including input voltage, total flow rate, mole ratio of methane to carbon dioxide, selective excitation of either reactant, and micro-arc formation, on product distribution and energy efficiency have been investigated. With an increase in the carbon dioxide-to-methane ratio the selectivity to carbon monoxide increased, and less coke formed. Micro-arc formation between excited methane and excited carbon dioxide increased the conversions of both methane and carbon dioxide and favored the production of carbon monoxide. The energy efficiency of the reaction reached a maximum at CH4/CO2 = 1 with micro-are formation, but it was minimized at CH4/CO2 = 1 when no micro-arc formed during the reaction. The reaction with micro-arc formation had a higher energy efficiency than that without micro-arc formation. (C) 2000 Academic Press.