The experiment was conducted to evaluate the performance of an upflow anaerobic sludge blanket (UASB) with granules for H2 production from a sucrose-rich synthetic wastewater at various substrate concentrations (5.33–28.07 g-COD/L) and hydraulic retention times (HRTs) (3–30 h) for over 3 years. The kinetics of H2 production was evaluated, and the sludge yield and endogenous decay coefficient of the H2-producing granules were estimated to be 0.334 g-VSS/g-COD and 0.004/h, respectively. Based on Gibbs free energy calculations, the formation thermodynamics of caproate, an important aqueous product, were analyzed. Experimental results show that the H2 partial pressure in biogas decreased with increasing substrate concentration, but was not sensitive to the variation of HRT in a range of 6–22 h. The H2 production rate increased with increasing substrate concentration, but decreased with increasing HRT. The H2 yield was in the range of 0.49–1.44 mol-H2/mol-glucose. Acetate, butyrate, caporate, and ethanol were the main aqueous products in the reactor, and their concentrations were dependent on both substrate concentration and HRT. An elevated substrate concentration resulted in a shift of fermentation from butyrate- to caporate-type in the reactor and the formation of caproate was dependent on the H2 partial pressure. The 3-year experimental results demonstrate that H2 could be produced continuously and stably from the acidogenic-granule-based UASB reactor. © 2006 Wiley Periodicals, Inc.