This paper presents thermodynamic analysis of commercial diesel with 50 ppm sulfur content for the three common modes of reforming operations. Thermodynamic analysis is done to get boundary data for carbon formation and to get the composition of various species for all modes and entire range of operations. For steam reforming operation, steam-to-carbon (S/C) ratio equal to or greater than 2 is required for carbon-free operation in entire temperature range (400-800 degrees C). However, selection of S/C ratio requires the balance between maximizing the hydrogen yield and minimizing the energy input both of which increase with increasing S/C ratio. For partial oxidation operation, O-2/C ratio of 0.75 is preferable to maximize hydrogen yield but carbon formation can occur if regions of reactor experience temperatures lower than 700 degrees C. In case of autothermal reforming, for carbon-free operation, temperature of 750 degrees C, O-2/C ratio in the range of 0.125-0.25 and S/C ratio greater than 1.25 and ideally 1.75 is recommended. However, enthalpy analysis indicates that it is not possible to reach to thermoneutral point at this condition so it is better to operate O-2/C ratio 0.25 or little higher with constant heat supply. A set of three independent reactions is proposed that along with element balance equations can adequately describe the equilibrium composition of six major species-H-2, CO2, CO, H2O, CH4, and C for the entire range of reforming operation. (C) 2009 Elsevier B.V. All rights reserved.