The optimisation of nitrogen sparging during fermentative hydrogen production was investigated. A N(2) sparging rate of 12 mL/min/L-liquid was observed to maximise the H(2) yield at approximately 2 mol H(2)/mol glucose converted, compared to an H(2) yield of approximately 1 mol H(2)/MOI glucose converted without any sparging. There was no significant increase in H(2) yield at sparging rates of 12-80 mL/min/L-liquid. The optimum sparging rate was lower than N(2) sparging rates examined in the past (>20 mL/min/L-liquid). To facilitate improved scale-up, the overall volumetric mass-transfer coefficients (K(L)a) for H(2) and CO(2) were measured and the relationship between the dimensionless Sherwood and Froude numbers was deter-mined. The optimal sparging rate occurred at a K(L)a value of 5.0 h(-1) for H(2), corresponding to a Sherwood number of 4800. By holding the Sherwood number constant upon scale-up, the full-scale K(L)a can be determined and the appropriate sparging rate can be determined from the corresponding Froude number. The benefits of operating at the optimum sparging rate, including minimising product hydrogen gas dilution and energy use, can thus be achieved in larger-scale systems. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.