The feasibility of hydrogen production from red algae was investigated. Galactose, the main sugar monomer of red algae, was readily converted to hydrogen by dark fermentation. The maximum hydrogen production rate and yield of galactose were 2.46 L H(2)/g VSS/d and 2.03 mol H(2)/mol galactose(added), respectively, which were higher than those for glucose (0.914 L H(2)/g VSS/d and 1.48 mol H(2)/mol galactose(added)). The distribution of soluble byproducts showed that H(2) production was the main pathway of galactose uptake. 5-HMF, the main byproduct of acid hydrolysis of red algae causes noncompetitive inhibition of H(2) fermentation. 1.37 g/L of 5-HMF decreased hydrogen production rate by 50% compared to the control. When red algae was hydrolyzed at 150 degrees C for 15 min and detoxified by activated carbon, 53.5 mL of H(2) was produced from 1 g of dry algae with a hydrogen production rate of 0.518 L H(2)/g VSS/d. Red algae, cultivable on vast tracts of sea by sunlight without any nitrogen-based fertilizer, could be a suitable substrate for biohydrogen production. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.