This investigation aimed to present a mathematical model for the behaviour of respiration rate and nitrate removal in an activated sludge sequencing batch reactor (SBR) with nitrification, denitrification and carbon oxidation. This model is based on the response of respiration rate (r) measured during nitrification and carbon oxidation and the nitrate removal rate during the post-denitrification period. For model validation, an SBR pilot plant (1 m(3)) receiving domestic wastewater was operated for three months, and the results were compared with the model. The respiration rate was used to calibrate several parameters of the model. The simulation values matched well with the measured respiration rate and nitrate removal. The model was able to predict respiration rate and denitrification in one cycle with parameters taken from the previous cycle. Parameter changes were followed during long-term operation, and significant variations were observed that were too complex to be predicted. It can be concluded that r is a good parameter for on-line monitoring of an activated sludge SBR with nitrification-denitrification processes.