In open-culture fermentation of lignocellulosic biomass, the gas composition in the fermentor headspace can suppress or favor certain fermentation pathways and thereby alter product concentrations. This study focuses on the effect of hydrogen and carbon dioxide concentrations in the headspace of open-culture fermentation of raw corn stover (80%) and chicken manure (20%). Four headspace gas compositions were evaluated at 2.05 x 10(5) Pa (abs): pure hydrogen, pure carbon dioxide, mixture of hydrogen and carbon dioxide (1:1), and pure nitrogen (control). Using pure hydrogen in the headspace increased the acetic acid equivalents by 13%, but did not change the product spectrum considerably. Using pure carbon dioxide in the headspace reduced the acetic acid equivalents by 4%, but shifted the acid spectrum from short toward medium-chain carboxylic acids. Using hydrogen: carbon dioxide (1:1) in the fermentor headspace increased the acetic acid equivalents by 29%.