Temperature, steam flow rate and coal/biomass ratio in the feedstock are the key factors that affect the performances co-gasification processes. In this study, co-gasification of sorghum biomass with high sulphur (5.61%) containing can lignite has been performed. The individual and simultaneous effects of temperature, water flow rate and coal/sorghum biomass ratio in feedstock were evaluated by Box-Behnken design (BBD) combined with response surface modelling (RSM). The significance of the quadratic models, independent variables and their interactions on the responses of carbon conversion efficiency (CCE, %), total gas, hydrogen, carbon monoxide, carbon dioxide, methane volumes (m(3)) and amount of H2S precipitated as CdS (g), were determined by analysis of variance (ANOVA). Temperature and coal/biomass ratio of the feedstock were found as the most effective process parameters on total gas yield and the composition of the produced gas and interaction effects were effective between temperature/water flow rate and temperature/biomass ratio. Coal/biomass ratio of feedstock was the only parameter that has a significant effect on the amount of H2S. Optimum conditions for maximum hydrogen production were determined with numerical optimization as temperature of 888 +/- 3 degrees C, water flow rate of 1.8 x 10(-3) m(3)/s and biomass ratio of the coal/biomass mixture of 25.9%. Results showed that sorghum biomass is a good candidate for high yield hydrogen production in co-gasification process with coal.