Dissolved H-2 and CO2 were measured by an improved manual headspace-gas chromatographic method during fermentative H-2 production with N-2 sparging. Sparging increased the yield from 1.3 to 1.8 mol H-2/mol glucose converted, although H-2 and CO2 were still supersaturated regardless of sparging. The common assumption that sparging increases the H-2 yield because of lower dissolved H-2 concentrations may be incorrect, because H-2 was not lowered into the range necessary to affect the relevant enzymes. More likely, N-2 sparging decreased the rate of H-2 consumption via lower substrate concentrations.