The software REBEKA was originally developed to estimate the impact of combined sewer overflows and stormwater discharges on receiving waters and to test measures with respect to reduce the impairments. It is widely used for the SWISS Master Plan of Urban Drainage (GEP). One aspect of the project «STORM» is to extend this software by implementing a stochastic routine. This enables it to handle uncertainties in model parameters and to quantify their impact on the results. The enhanced stochastic version, REBEKA II, is presented in this paper. This version handles, in addition to parameter uncertainties, impacts by total suspended solids.