Storm retention tanks are used widely for regulating pollution caused by combined sewer overflows, a notably transient, dynamic phenomenon. Nevertheless, few dynamic models of the process have hitherto been proposed. A conceptual model for simulation of the dynamic performance of a storm tank is presented. Four modes of behaviour are identified: fill, draw, dynamic sedimentation and quiescent settling. Discussion is given of the problems associated with the model, in particular the characterization of spatial non-uniformities in pollutant concentrations. Simulation results are presented for the application of the model to a test storm disturbance based on field data taken from the Norwich Sewage Works in eastern England. The disturbance comprises the response of a sewer network to a high-intensity, short-duration precipitation event, with a significant transient increase in suspended solids concentration and a transient depression in ammonium-N concentration. The impact of the storm tank returns on primary clarifier performance is found to be almost negligible. Some tests of the sensitivity of the storm tank model to changes of its parameters, such as particle settling velocity, are discussed.