The effect of three limiting nutrients, calcium pantothenate, vitamin B(12) and cobalt chloride (CoCl(2)), on syngas fermentation using "Clostridium ragsdalei" was determined using serum bottle fermentation studies. Significant results from the bottle studies were translated into single- and two-stage continuous fermentor designs. Studies indicated that three-way interactions between the three limiting nutrients, and two-way interactions between vitamin B(12) and CoCl(2) had a significant positive effect on ethanol and acetic acid formation. In general, ethanol and acetic acid production ceased at the end of 9 days corresponding to the production of 2.01 and 1.95 gL(-1) for the above interactions. Reactor studies indicated the three-way nutrient limitation in two-stage fermentor showed improved acetic acid (17.51 g g(-1) cells) and ethanol (14.74 g g(-1) cells) yield compared to treatments in single-stage fermentors. These results further support the hypothesis that it is possible to modulate the product formation by limiting key nutrients during C. ragsdalei syngas fermentation. (C) 2011 Elsevier Ltd. All rights reserved.