A start-up experiment was performed in a laboratory-scale, upflow anaerobic sludge blanket (UASB) reactor using seed sludge from a domestic waste treatment plant at 3.8-33.3 gCOD l(-1) day(-1) loading rates. Analysis over the height of the reactor with time showed that the VSS in the reactor was initially differentiated into active and non-active biomass at increasing gas production and upflow velocities, and specific update rates of the volatile fatty acids (VFA) components were pronounced at the bottom 10% of the reactor. During start-up, specific methanogenic activity and chemical oxygen demand (COD) uptake rate increased from 0.075 to 0.75 gCOD-CH4 (gVSS)(-1) day(-1) and from 0.08 to 0.875 gCOD removed (gVSS)(-1) day(-1), respectively. When seed sludge from a distillery waste treatment plant was used, improved performance due to a predominance of active biomass was evident when the loading Fate was increased from 9.4 to 28.7 gCOD l(-1) day(-1). The Proposed start-up evaluation is an effective tool to successfully monitor performance of UASB reactors. (C) 2008 Elsevier Ltd. All rights reserved.