A new kinetic model is developed to describe the microbial storage process under anoxic conditions in activated sludge. The accumulation of nitrite in denitrification is taken into account in this model. The formation of storage polymers under anoxic conditions is coupled with energy generation and anoxic respiration, and their formation rate is found to be proportional to the substrate utilization rate. The sensitivity of the nitrogen uptake rate toward the stoichiometric and kinetic coefficients of the established model is analyzed by using an "absolute-relative" sensitivity function implemented in software AQUASIM. Batch denitrifying experiments were conducted to calibrate and validate the established model. The model is compared with ASM1 and ASM3, the two commonly used activated sludge models. Results show that the kinetic model established in this work is able to better describe the microbial storage processes under anoxic conditions. (C) 2008 Elsevier Ltd. All rights reserved.