Alkaline pre-treatment has been widely applied to lignocellulosic biomass but the tested conditions are quite variable in literature. Results are also quite scattered even when similar substrates are compared. Therefore the aim of this study was to test different alkaline dosages (4% and 10% gNaOH/gTS), temperatures (40 degrees C and 55 degrees C), and contact times (12 h and 24 h) in order to investigate the influence of the pretreatment conditions on the structural features and methane production from ensiled sorghum forage. This study confirms the positive effect of NaOH pre-treatment on fibre reduction, total organic carbon and proteins solubilisation, and thereafter the anaerobic degradability of ensiled sorghum forage. An increase in methane yield, with respect to untreated sample (from 8% to 19%), was observed at all pretreatment conditions tested. Nevertheless, no significant differences on methane yield were observed by varying NaOH dosage, temperature, and contact time. The increase of sodium hydroxide dosage led to an increase of the soluble total organic carbon (TOC) (from 12% to 29%) and proteins (from 56% to 72%), at each temperature and contact time tested. By increasing the NaOH dosage, a reduction of hemicelluloses (from 37% to 70%) and lignin contents (from 26% to 70%), and an increase of the anaerobic digestion kinetics (with a maximum increase of 43% for samples treated at 55 degrees C for 24 h), were also observed. Finally, the anaerobic digestion kinetics were improved with the increase of contact time (up to 13%) and temperature (up to 20%). (C) 2012 Elsevier B.V. All rights reserved.