A structured model for sequencing batch reactors, which was developed earlier and tested successfully against a number of experimental data sets, is used in this study to investigate the sensitivity of model predictions to some of the system kinetic parameters for a wide range of parameter values. The results obtained reveal the relative importance of the various parameters. The parameter K-s has insignificant effects on the effluent COD concentration; although its effect on the intermediate COD concentration is appreciable. The parameters alpha(1), mu(m), K-p, K-x, K-h and B-h have been shown to affect considerably both the intermediate and effluent COD concentrations. Since the effluent COD concentration is one of the prime state variables in the design of wastewater treatment reactors, special attention should be given to these parameters when such a model is used for design and control purposes.