The food processing industry produces highly concentrated, carbohydrate-rich wastewaters, but their potential for biological hydrogen production has not been extensively studied. Wastewaters were obtained from four different food-processing industries that had chemical oxygen demands of 9 g/L (apple processing), 21 g/L (potato processing), and 0.6 and 20 g/L (confectioners A and B). Biogas produced from all four food processing wastewaters consistently contained 60% hydrogen, with the balance as carbon dioxide. Chemical oxygen demand (COD) removals as a result of hydrogen gas production were generally in the range of 5-11 %. Overall hydrogen gas conversions were 0.7-0.9 L-H-2/L-wastewater for the apple wastewater, 0.1 L/L for Confectioner-A, 0.4-2.0 L/L for Confectioner B, and 2.1-2.8 L/L for the potato wastewater. When nutrients were added to samples, there was a good correlation between hydrogen production and COD removal, with an average of 0.10 +/- 0.01 L-H-2 /g-COD. However, hydrogen production could not be correlated to COD removal in the absence of nutrients or in more extensive in-plant tests at the potato processing facility. Gas produced by a domestic wastewater sample (concentrated 25x) contained only 23 +/- 8% hydrogen, resulting in an estimated maximum production of only 0.01 L/L for the original, non-diluted wastewater. Based on an observed hydrogen production yield from the effluent of the potato processing plant of 1.0L-H-2/L, and annual flows at the potato processing plant, it was estimated that if hydrogen gas was produced at this site it could be worth as much as $65,000/year. (c) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.