The degradation of a non-inhibitory substrate (sucrose) in upflow anaerobic sludge bed (UASB) reactors with different superficial flow velocites (u(s)) was performed to generate experimental data. Additionally, a kinetic model accounting for the mass fraction of methanogens (f) and granule size distribution in UASB reactors is also proposed. At the volumetric loadings of 2.65-21.16 g COD dm(-3) day(-1), both the COD removal efficiency and granule size of the UASB reactors increase with increasing u(s). The f values determined experimentally increase from 0.13-0.24 to 0.27-0.43 if the volumetric loading is increased from 2.65 to 5.29 g COD dm(-3) day(-1). With a further increase in volumetric loading, the f values decline because of the accumulation of volatile fatty acids (VFAs). The predicted residual concentrations of VFAs and COD are in fairly good agreement with the experimental data. From the calculated effectiveness-factor values, the influence of mass transfer resistance of the substrate sucrose on the overall substrate removal rate should not be neglected. From parametric sensitivity analyses together with the simulated concentration profiles, methanogenesis is the rate-limiting step. (C) 2003 Society of Chemical Industry.