Clostridium ljungdahlii was grown on syngas of varying composition to investigate how syngas blends used in fermentation can be tailored toward desired product formation. Experiments were conducted with liquid batch cultures exposed to the same volume of gas with varying ratios of hydrogen to carbon monoxide (H-2/CO). Formation of acetate was favored by higher concentrations of hydrogen in the headspace. The maximum acetate concentration of 35.21 +/- 0.84 mM was obtained using an H-2/CO ratio of 2.0. Generation of more reduced metabolites was favored by greater carbon monoxide concentrations in the headspace. Maximum ethanol (7.53 +/- 0.58 mM) and 2,3-Butanediol (5.20 +/- 0.79 mM) concentrations were achieved under an H-2/CO ratio of 0.5. Linear relationships were obtained in that describe the effect of the syngas H-2/CO ratios on acetate, ethanol, and 2,3-BD formation, respectively.