Hydrogen yields in the syngas produced from non-catalytic biomass gasification are generally low. The hydrogen fraction, however, can be increased by converting CO, CH, higher hydrocarbons, and tar in a secondary reactor downstream. This paper discusses thermodynamic limits of the synthesis gas upgrading process. The method used in this process is minimization of Gibbs free energy function. The analysis is performed for temperature ranges from 400 to 1300 K, pressure of 1-10 atm. (0.1-1 MPa), and different carbon to steam ratios. The study concludes that to get optimum H-2 yields, with negligible CH4 and coke formation, upgrading syngas is best practiced at a temperature range of 900-1000 At these temperatures, H-2 could be possibly increased by 43-124% of its generally observed values at the gasifier exit. The analysis revealed that increasing steam resulted in a positive effect. The study also concluded that increasing pressure from 1 to 3 atm can be applied at a temperature >1000 K to further increase H-2 yields. (C) 2009 Elsevier Ltd. All rights reserved.