Adding O-2 into biogas to achieve partial oxidation and CO2 mixed reforming can not only increase H-2 + CO concentration, but also reduce energy cost for H-2 production. In this study, optimized mixed reforming of biogas with O-2 addition in spark-discharge plasma was pursued in combination with thermodynamic-equilibrium calculation. With respect to mixed reforming of biogas with O-2 addition in spark-discharge plasma, combination coefficients of independent reactions were given to quantitatively evaluate the mixed extent at various O-2/(CH4-CO2) ratios. Compared thermodynamic-equilibrium with experimental results, it can be concluded that the optimal O-2/(CH4-CO2) ratio for optimized mixed reforming of biogas in spark-discharge plasma was about 0.7. When total-carbon conversion was relatively high (>75%), H-2 + CO concentration on wet basis was the highest and energy cost for H-2 production was the lowest at O-2/(CH4-CO2) = 0.7, and their experimental results were closest to their thermodynamic-equilibrium values. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.