In this study, synthesis gas production in an AC electric gas discharge of methane and air mixtures at room temperature and ambient pressure was investigated. The objective of this work was to understand how the CH4/O-2 feed mole ratio, ethane added, diluent gas, residence time, input power, applied frequency, and waveform, affected methane and oxygen conversions, product selectivities, and specific energy consumption. Methane and oxygen conversions increased with input power but decreased with increasing CH4/O-2 feed mole ratio, flow rate, and gap distance. The experiments were performed at the frequency and power in the range of 200-700 Hz and 8-14 W, respectively, while the residence times were varied from 0.06 to 0.46 s. This study confirms that active oxygen is an important factor in enhancing methane conversion and energy efficiency in a discharge reactor. Ethane is the primary product that forms at short residence times and low energies. Methane conversion dropped dramatically but oxygen conversion increased with addition of ethane to the feed gas. Sinusoidal and square waveforms gave negligibly different results. Current was constant with varying CH4/O-2 ratio and flow rate, but increased with increasing power and with decreasing gap distance and frequency. It was shown that the best condition was at 300 Hz and at the highest power used in each condition, since the maximum methane and oxygen conversions and synthesis gas selectivity as well as lowest specific energy consumption were found both with and without ethane in the feed gas. The minimum specific energy consumption, found at 300 Hz, were 21 and 14 eV/m(c) for the CH4/air system and the CH4/air/C2H6 system, respectively. When studying the effect of residence time by varying the flow rate, the minimum energy consumption of 21 eV/m(c) was found at 0.12 and 0.23 s. For any given input power or frequency, the CH4/air system had a higher specific energy consumption than the CH4/air/C2H6 system. Less energy was consumed to convert methane under the plasma environment with nitrogen as a diluent compared to helium, indicative of a third body effect.