In order to probe into the role of water in the process of natural gas generation, lignite from the Shenshan coal field of the Ordos Basin, China, was selected for pyrolytic simulation experiments under hydrous and anhydrous conditions for the purpose of comparison. The results showed that water was involved in the chemical reactions that took place during gas generation, and enhanced the productivity of CO 2 and H 2 . Moreover, the involvement of water led to an increase in the productivity of hydrocarbons in a certain degree, especially methane at high temperatures. The Ξ΄D values of CH 4 become linearly heavier with increasing productivity of methane as observed in the two series of experiments, and the Ξ΄D values of CH 4 in the hydrous experiments are about 35β° lighter than those obtained in the anhydrous experiments in the case of the same productivity of CH 4 . The fact that the Ξ΄ 13 C values of CO 2 have become lighter in the hydrous experiments is strong evidence for the involvement of water in the chemical reactions during gas generation. Such trend of variations appears to have been caused by the conversion of methylene carbon to carboxyl carbon by water/ alkene interaction in hydrous condition.