Changes in oxidation-reduction potential (ORP) values during Escherichia coli Delta hyaB Delta hybC double mutant lacking of large subunits of hydrogenases 1 (Hyd-1) and 2 (Hyd-2) and effects of redox reagents were studied upon glucose fermentation at acidic (5.5) and alkaline (7.5) pHs. H-2 production yield was determined by the readings of platinum and titanium-silicate electrodes. Specific growth rate of mutant was inhibited by similar to 1.3 fold at pH 5.5 in the presence of membrane-permeating reducing agent DL-dithiothreitol (DTT) and impermeable oxidant potassium ferricyanide (K-3[Fe(CN)(6)]), whereas at pH 7.5 the reagents had no significant effects. H-2 production was stimulated by similar to 1.4 fold with mutant compared to the wild type at pH 7.5. Moreover, 3 mM DTT stimulated H-2 production yield by similar to 2 and similar to 4 fold during bacterial log growth phase at pH 5.5 and pH 7.5, respectively. While at pH 5.5 1 mM K-3[Fe(CN)(6)] completely inhibited H-2 formation by mutant as well as wild type resulting in alternation of ORP to positive values (200 +/- 10 mV) at the end of log growth phase. At pH 7.5H(2) formation by the wild type was inhibited by similar to 1.2 fold, while in the case of mutant H-2 formation was completely inhibited, but resulting in decrease of ORP to negative values (-270 +/- 12 mV) at the end of log growth phase. These findings point out the effects of lacked Hyd-1 and Hyd-2 both on H-2 production by E. coli and indicate the importance of reductive conditions for enhanced H-2 production as well. Copyright (c) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.