In this work the effect of the undissociated fermentation products concentration on the biomass growth and bio-hydrogen generation by acidogenic fermentation has been studied. A mixed culture enriched by biokinetic control was used to perform parallel batch experiments at different substrate concentrations. From these experiments it was observed that the biomass growth rate and yield significantly depended on the fermentation products concentration reached. The concentration of undissociated fermentation products causing significant effects on the growth rate were found to be 10 mM, for the shift from the typical exponential trend to a linear one, and 30 mM for the total inhibition of the biomass growth. Regarding the bio-hydrogen generation it was observed that its generation rate was linked to the biomass concentration, being the average yield not affected. However, when working at very high concentration of undissociated fermentation products, values higher than 30 mM, it was observed a slight decrease of the generation rate which could be caused by a change in the metabolic route used to ferment the substrate. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.