A model was developed to describe the performance of sequencing batch reactors. The model was structured upon four processes (substrate growth depending, product growth depending, hydrolysis, and decay processes). The model parameters, two parameters, were evaluated using one set of experimental data obtained from the literature. The model was then used without adjusting any parameters to predict five other experimental data sets. The excellent predictive nature of model was validated for a wide range of fill to react period ratio (13:31) and also for a wide range of COD loading (340:3300 mg/l).