In this study, an efficient strategy to increase the hydrogen yield from cassava stillage (CS), by its co-digestion with organic wastes, was developed. The effects of different co-substrates on hydrogen yield were evaluated and compared, with increases obtained in all co-digestion reactors. The maximal hydrogen yield was achieved by CS co-digestion with cassava excess sludge (CES), which resulted in a 46% increase over the yield from CS alone. Improved hydrolysis and acidification performances, and thus increased hydrogen production, were additional benefits of co-digestion, especially with CES. Also, the presence of a co-substrate promoted butyrate over lactate generation. The most influential advantage of co-substrate addition was the enhanced pH buffering capacity, which correlated linearly with hydrogen yield (R-2 = 0.98). Moreover, the co-substrates addition resulting in more favorable carbohydrate-COD/protein-COD ratio, C/N and C/P ratios, all of which also correlated linearly with hydrogen yield (R-2 = 0.86, 0.71, and 0.75 respectively). Polymerase chain reaction-denaturing gradient gel electrophoresis analysis of the role of microorganisms indicated that the addition of even small amounts of co-substrate altered the bacterial communities within the reactors, enriching the proportion of hydrogen-producing bacteria. A species detected only in the co-digestion reactors was shown to be related to Clostridium cellulosi, which is an efficient hydrogen-producer, while some bacteria not involved in hydrogen production and present in the CS alone reactor were not detectable in any of the co-substrate-containing reactors. Copyright (c) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.