Significant amounts (>36 million m(3)/day) of coalbed methane (CBM) are currently being extracted from coal beds in the Paleocene Fort Union Formation of the Powder River Basin of Wyoming and Montana. Information on processes that generate methane in these coalbed reservoirs is important for developing methods that will stimulate additional production. The chemical and isotopic compositions of gas and ground water from CBM wells throughout the basin reflect generation processes as well as those that affect water/rock interaction. Our study included analyses of water samples collected from 228 CBM wells. Major cations and anions were measured for all samples, delta D(H2O) and delta(18)O(H2O) were measured for 199 of the samples, and delta D(CH4) of gas coproduced with water was measured for 100 of the samples. Results show that (1) water from Fort Union Formation coal beds is exclusively Na-HCO(3)-type water with low dissolved SO(4) content (median<1 mg/L) and little or no dissolved oxygen (<0.15 mg/L), whereas shallow groundwater (depth generally<120 m) is a mixed Ca-Mg-Na-SO(4)-HCO(3) type; (2) water/rock interactions, such as cation exchange on clay minerals and precipitation/dissolution of CaCO(3) and SO(4) Minerals, account for the accumulation of dissolved Na and depletion of Ca and Mg; (3) bacterially-mediated oxidation-reduction reactions account for high HCO(3) (270-3310 mg/L) and low SO(4) (median <0.15 mg/L) values; (4) fractionation between delta D(CH4) (-283 to -328 per mil) and delta D(H2O) (-121 to -167 per mil) indicates that the production of methane is primarily by biogenic CO(2) reduction; and (5) values of delta D(H2O) and delta(18)O(H2O) (-16 to -22 per mil) have a wide range of values and plot near or above the global meteoric water line, indicating that the original meteoric water has been influenced by methanogenesis and by being mixed with surface and shallow groundwater. Published by Elsevier B.V.