This study aimed to improve the productivity of dark fermentative hydrogen production from organic waste. An anaerobic sequencing batch reactor was used for hydrogen fermentation and it was fed with food waste (VS 4.4 +/- 0.2% containing 27 g carbohydrate-COD/L) at various CO2 sparging rates (40-120 L/L/d), hydraulic retention times (HRTs; 18-42 h), and solid retention times (SATs; 18-160 h). CO2 sparging increased the H-2 productivity by 5-36% at all the examined conditions, confirming the benefit of the replacement of headspace gas by CO2. The maximum H-2 production was obtained by CO2 sparging at 80 L/L/d, resulting in the H-2 productivity of 3.18 L H-2/L/d and the H-2 yield of 97.3 mL H-2/g VSadded. Increase of n-butyrate and isopropanol yields were concurrent with the enhanced H-2 yield by CO2 sparging. Acidogenic efficiency, the sum of H2, organic acid, and alcohol, in the CO2-sparged reactor ranged from 47.9 to 56.0%, which was comparable to conventional acidogenesis. Thermodynamic analysis confirmed that both CO2 sparging and CO2 removal were beneficial for H-2-producing reactions, but CO2 sparing has more profound effect than CO2 removal on inhibiting H-2-consuming reactions. Copyright (c) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.