Hydrogen-producing granules with an excellent settling ability were cultivated in an upflow anaerobic sludge blanket reactor treating a sucrose-rich synthetic wastewater. The physicochemical characteristics of granules were evaluated in this study. The mature granules had a diameter ranging from 1.0 to 3.5 mm and an average density of 1.036 ± 0.005 g/mL, whereas they had good settling ability and a high settling velocity of 32–75 m/h. The low ratio of proteins/carbohydrates for the extracellular polymeric substances (EPS) in the granules suggests that carbohydrates rather than proteins, might play a more important role in the formation of the H2-producing granules. The contact angle of the mature granules, 54 ± 2°, was larger than that of the seed sludge (38 ± 2°), indicating that the microbial cells in the H2-producing granules had higher hydrophobicity. The granules had fractal nature with a fractal dimension of 1.78. Their porosities were in the range of 0–0.70, and increased with increasing granule size. The ratios between the observed and predicted settling velocities by Stokes' law were in a range of 1.00–1.50, and the fluid collection efficiency of the granules ranged from 0 to 0.19, indicating that their permeabilities were lower and that there was little advective flow through their interior. Experimental results also suggest that molecular diffusion appeared to play an important role in the mass transfer through the H2-producing granules. © 2006 Wiley Periodicals, Inc.