This study was carried out to document the source and stable isotopic characteristics of coal bed gas and associated coal bed water samples of the Raniganj Basin, West Bengal, India. The Raniganj Basin is one of the largest coal bed gas-producing sites in India. However, the gas isotopic composition of this basin was not well investigated until now. The exploration has been focused on mature coals with a high content of assumed thermogenic methane. Interestingly, the present study has revealed that although the gas is thermogenic, there is a significant effect of microbial activities in gas generation. The gas explored from this Basin contains almost 98-99% methane (CH4) in general with very little portions of other hydrocarbons and thus, this gas can be considered as "dry gas". It is consistent also with late-stage microbial alteration of thermogenic gases. The delta C-I3 and delta D values of CH4 range from -47.4 to -49.7 parts per thousand and -207 to -211 parts per thousand respectively across the three wells as well as from -48.8 to -50.1 parts per thousand and -209 to -211 parts per thousand respectively in the gas samples collected from gas gathering station. The Whiticar style cross plot of these two parameters reveals that the gas samples have mixed thermogenic and biogenic sources and the microbial gas was generated through the CO2 reduction pathway rather than the acetate fermentation pathway. The difference in SD values of water and methane samples [delta D (H2O-CH4)], that varies from 173 to 175 parts per thousand, may also exhibit that the biogenic methane was produced through reduction of CO2. The stable hydrogen and oxygen isotopic signatures of formation water samples (delta D and delta O-18-H2O) stand with the mixed origin of the gas. The cross-plot of these two parameters reveals that the water samples lie almost along the Global Meteoric Water Line (GMWL), which may suggest the combined effect of methanogenesis due to mixing of coal seam formation water with modern meteoric water. In summary, the produced coal bed gas from the Raniganj Basin, although being largely thermogenic in origin, has been influenced by secondary microbial alteration.