A mathematical model describing the combined anaerobic degradation of complex organic material, such as manure, and a lipid containing additive, such as olive oil mill effluents, has been developed based on a model previously described (Angelidaki et al. 1993). The model has been used to simulate anaerobic codigestion of cattle manure together with olive oil mill effluent (OME) and the simulations were compared with experimental data. Simulation data indicated that lack of ammonia, needed as nitrogen source for synthesis of bacterial biomass and as an important pH buffer, could be responsible for the problems encountered when anaerobic degradation of OME alone is attempted. It was shown that the amount of nitrogen needed to obtain a stable degradation of OME can be provided by manure during codigestion of OME and manure.