Fermentative hydrogen from seaweed is a potential biofuel of the future. Mannitol, which is a typical carbohydrate component of seaweed, was used as a substrate for hydrogen fermentation. The theoretical specific hydrogen yield (SHY) of mannitol was calculated as 5 mol H-2/mol mannitol (615.4 mL H-2/g mannitol) for acetic acid pathway, 3 mol H-2/mol mannitol (369.2 mL H-2/g mannitol) for butyric acid pathway and 1 mol H-2/mol mannitol (123.1 mL H-2/g mannitol) for lactic acid and ethanol pathways. An optimal SHY of 1.82 mol H-2/mol mannitol (224.2 mL H-2/g mannitol) was obtained by heat pre-treated anaerobic digestion sludge under an initial pH of 8.0, NH4Cl concentration of 25 mM, NaCl concentration of 50 mM and mannitol concentration of 10 g/L. The overall energy conversion efficiency achieved was 96.1%. The energy was contained in the end products, hydrogen (17.2%), butyric acid (38.3%) and ethanol (34.2%). (C) 2015 Elsevier Ltd. All rights reserved.