A thermodynamic model is developed to estimate equilibrium composition for supercritical water gasification (SCWG) of biomass. With a local optima solver and a global optima solver of LINGO, the algorithm based on Peng-Robinson EoS formulations and direct Gibbs free energy minimization can guarantee the convergence to the correct solution. Results are given for the supercritical water processes including supercritical water reforming of methanol, supercritical water gasification of glucose, catalyzed supercritical water gasification of cellulose and supercritical water gasification of real biomass. Model predictions are compared with various experimental measurements and the agreement is generally satisfying and therefore the correctness of the proposed model is demonstrated. Significant improvements of the comparison are obtained by analyzing reaction network and controlling steps of these processes and accounting them into calculation. (c) 2004 Elsevier B.V. All rights reserved.