Hydrogen production by dark fermentation may suffer of inhibition or instability due to pH deviations from optimality. The co-fermentation of promptly degradable feedstock with alkali-rich materials, such as livestock wastes, may represent a feasible and easy to implement approach to avoid external adjustments of pH. Experiments were designed to investigate the effect of the mixing ratio of fruit-vegetable waste with swine manure with the aim of maximizing biohydrogen production while obtaining process stability through the endogenous alkalinity of manure. Fruit-vegetable/swine manure ratio of 35/65 and HRT of 2 d resulted to give the highest production rate of 3.27 +/- 0.51 L(H2) L(-1) d(-1), with a corresponding hydrogen yield of 126 +/- 22 mL(H2) g(-1) (VS-added) and H(2) content in the biogas of 42 +/- 5%. At these operating conditions the process exhibited also one of the highest measured stability, with daily productions deviating for less than 14% from the average. (C) 2011 Elsevier Ltd. All rights reserved.