The interaction between bacterial species is of great importance for H-2 production using microbial consortia or non-sterile conditions. Sulfate reducing bacteria were found in anaerobic starch-hydrolyzing consortium and their inhibitory effect on the following H2 photoproduction by purple nonsulfur bacteria was shown. This inhibition was clearly demonstrated in the mixed culture of Rhodobacter sphaeroides and Desulfomicrobium baculatum using the synthetic medium. This effect was conditioned by sulfide production rather than H-2 consumption or competition for organic substrate. Actually, the addition of equivalent sulfide concentration brought about the similar effects: inhibition of H-2 production without growth inhibition, cells aggregation, and the increase of carbohydrate content as an alternative way of expenditure of organic acids. In the long-term experiments the average sulfide concentration of about 0.3 mM was detrimental while in short-terms the H-2 production was not inhibited even at 3.2 mM. The protective effect of molybdates against sulfate reducers and sulfide was discussed. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.