In the presence of surfactant sodium dodecylbenzene sulfonate (SDBS) the hydrolysis products accumulation and the short-chain fatty acids (SCFA) production during waste activated sludge fermentation under mesophilic and thermophilic conditions was compared with that at room temperature. In order to understand the mechanism of significant amounts of mesophilic and thermophilic hydrolysis products and SCFA observed in the presence of surfactant, the kinetic models at different SOBS dosages were developed. It was found that SDBS increased the mesophilic and thermophilic hydrolysis rate significantly, and the maximum specific utilization of hydrolysis products Increased at low SDBS and decreased at high one However, the observed maximum specific utilization of SCFA decreased seriously with SOBS increase. In the presence of SOBS the decay rate of acidogenic bacteria not only was lower than that in the absence of SOBS but decreased with the increase of SOBS under either mesophilic or thermophilic conditions (C) 2010 Elsevier Ltd All rights reserved