The effects of sulfuric acid, acetic acid, aqueous ammonia, sodium hydroxide, and steam explosion pretreatments of corn stalk on organic acid production by a microbial consortium, MC1, were determined. Steam explosion resulted in a substrate that was most favorable for microbial growth and organic acid productions. The total amounts of organic acids produced by MC1 on steam exploded, sodium hydroxide, sulfuric acid, acetic acid, and aqueous ammonia pretreated corn stalk were 2.99, 2.74, 1.96, 1.45, and 2.21 g/l, respectively after 3 days of fermentation at 50 degrees C. The most prominent organic products during fermentation of steam-exploded corn stalks were formic (0.86 g/l), acetic (0.59 g/l), propanoic (0.27 g/l), butanoic (0.62 g/l), and lactic acid (0.64 g/l) after 3 days of fermentation: ethanol (0.18 g/l), ethanediol (0.68 g/l), and glycerin (3.06 g/l) were also produced. These compounds would be suitable substrates for conversion to methane by anaerobic digestion. (C) 2011 Published by Elsevier Ltd.