A detailed thermodynamic analysis of equilibrium products was conducted for natural gas fuel processing under simultaneous partial oxidation (POX) and steam reforming (SRF) processes. Fuel processing by this arrangement requires no extra heat from external sources and the required thermal energy for endothermic SRF is obtained from the exothermic POX; assuming adiabatic POX and SRF reactions. The equilibrium product composition and the equilibrium temperature are both dependent on the air-fuel ratio and water-fuel ratio. A computer program developed at NASA Lewis Research Center was adopted and modified in this study for calculating the complex chemical equilibrium composition. With different combinations of the molar air-fuel ratio and molar water-fuel ratio in the fuel-air-water reaction system, it is possible to obtain a maximum hydrogen yield with minimum productions of carbon monoxide and solid carbon. Results showed that an optimal hydrogen yield of 36.3-36.6% can be achieved when the molar air-fuel ratio and molar water-fuel ratio are set to 3.5 and 2.5-4, respectively, and one mole of natural gas can produce 2.19-2.22 moles of hydrogen. Under this condition, the formations of carbon monoxide and residue methane are 2.24-4.38% and 0.55-0.96%, respectively; while the formation of solid carbon is effectively suppressed to zero. The product＇s temperature under the assumption of adiabatic reaction corresponding to this condition is 820-871 K. (C) 2000 International Association for Hydrogen Energy.