The performance of sequence batch reactor (SBR) was studied under four different air fluxes. Special attention was paid to the operating characteristics of SBR under limited aeration or low dissolved oxygen (DO) conditions. At the air flux of 40 l/h, COD and NH4+-N had been removed just before the cycle was over, and during the cycle DO was about 0.5 mg/l most of the time. Operational parameters, such as DO, ORP and pH, were monitored during the whole cycle. The effect of these parameters on the removal efficiency of COD and NH4+-N was discussed. (C) 2004 Elsevier Ltd. All rights reserved.