The dispersion behavior of an upflow anaerobic sludge blanket (UASB) reactor for acidogenesis was investigated. and its flow patterns were simulated. Compared with the single-zone axial-dispersion model (with squared difference from 130 to 450), a two-zone axial-dispersion model (with squared difference from 1 to 7) was found to be more appropriate for simulating the dispersion characteristics of the acidogenic UASB reactor. A sensitivity analysis of the key model parameters was performed, and the axial dispersion coefficient was identified as the most important factor in the dispersion modeling of the reactor, implying that the acidogenic UASB reactor was potentially dispersion-control led. The How patterns, including mixed and plugging degrees, at different hydraulic loading rates were evaluated. The ratio of plug volume to mixed volume (I-p/I-m) decreased with the increasing specific gas velocity (U-g(sp)) when the U-g(sp) value was low, but increased markedly with the increasing U-g(sp) after the U-g(sp) exceeded 0.7. (C) 2007 Elsevier B.V. All rights reserved.