The relationship between various unsupported molybdenum carbides and their activity toward methane reforming at 973 K and 1 atm was studied. Unsupported molybdenum carbides catalyzed the formation of hydrogen in high selectivity, forming ethylene and ethane rather than benzene as the carbon-containing products. eta -Mo3C2, which was nitrided at 973 K and subsequently carbided at 1173 K, was more active than both alpha -MoC1-x and beta -Mo2C in methane decomposition, forming hydrogen in high selectivity. alpha -MoC1-x and gamma -Mo2N were transformed to eta -Mo3C2 in the bulk structure during methane reforming at 973 K. This transformation caused a significant increase in the turnover frequency of methane reforming. eta -molybdenum carbide was also formed during CH4-TPR Of gamma -Mo2N at 788 K. The linear relationship between the amount of il-carbide determined through H-2-TPR of the catalysts and the methane disappearance rate revealed that eta -Mo3C2 is the active species for methane reforming. From the XPS analysis, Mo-0 was the dominant molybdenum species for the eta -Mo3C2 catalysts.