Granule-based biological H-2 production processes are gaining great popularity in recent years. An efficient and stable operating of such systems relies heavily on the performance of the H-2-producing granules (HPGs), which possess many unique properties compared with floc sludge and methanogenic granules. Hence, a full understanding of the sludge characteristics is essential. Especially, the physicochemical properties of HPGs may provide useful information for effective evaluation of system status. This review offers a systematical introduction of the physicochemical properties of HPGs, including size, morphology, settling velocity, permeability, rheology, surface charge, hydrophobicity and extracellular polymeric substances (EPS). We also analyze the relationships between these physicochemical factors and the system performance, and discuss the remaining challenges and future implications for sludge characterization and process monitoring. This work may facilitate a better understanding of granule-based biological H-2 production processes and offer a basis for timely process monitoring and manipulation. (C) 2011 Elsevier Ltd. All rights reserved.