In this paper we present an approach to describe organic degradation within immobile water regions of Municipal Solid Waste (MSW) landfills which is best described by the term “gray box” model. We use a simplified set of dominant (bio)chemical and physical reactions and realistic environmental conditions. All equations, relationships and inhibitions are based on semi-empirical or fundamental relationships which have proven to be applicable in the peer reviewed literature. As much as possible parameter values are obtained from thermodynamic relationships and literature, a minimum set of parameters needs to be identified by fitting. Modeled emissions of biogas, Volatile Fatty Acids (VFA), pH and ammonium are in good agreement with the experimental data. The model is able to describe the delay in methanogenic activity, the dynamics in pH and the partial pressures in the gas phase. Results from a parameter optimization indicate a missing transport limitation. Our aim is to include the model as a bio(geo)chemical module in other models because of it's computational efficiency.