A model is presented for predicting the composition (H(2)O, CO(2), CH(4), H(2), CO, O(2) and C(2)H(6)) in the C-O-H fluid system under high temperatures and pressures found in the Earth's mantle. The model is based on a molecular dynamic equation of state, statistical mechanics calculations and non-stoichiometric global free-energy minimization. Although the model is not fitted to experimental data on C-O-H speciation, it does accurately reproduce these datasets and should extrapolate at least to the depths of similar to 80-220 km. The model results suggest that (1) in the upper cratonic mantle, H(2)O is the dominant fluid species in the C-O-H fluid system; (2) the abundance of CO(2) increases with decreasing depth, the trend of CH(4) is just the opposite; (3) the boundary between lithosphere and asthenosphere generally divides fluid systems into H(2)O-CH(4)+ minor species and H(2)O-CO(2)+ minor species, respectively; (4) it is entirely possible to generate methane and ethane and possibly other hydrocarbons under mantle conditions, confirming previously experimental results. (c) 2009 Published by Elsevier Ltd.