A model for simulation of substance flows in urban sewer systems has been developed in the computer programme package MATLAB/Simulink. The model is called SEWSYS and it handles fluxes of substances within an urban catchment. SEWSYS keeps track of each substance; where it originates and where it finally ends up. Some of the substances included are water, phosphorus, nitrogen, heavy metals and PAH. SEWSYS consists of a stormwater module, sanitary wastewater module and a wastewater treatment plant with nitrogen reduction. Verifications have been carried out both for each component separately and for the model as a whole. The results show that the modelling approach is valid, but further enhancements should be made