The present thesis develops a mathematical model for the assessment of CO₂ mitigation costs of biomass CHP systems. A scientific evaluation method is investigated which is applicable for different engine concepts and biofuels. The model relies on the life cycle analysis of costs and CO₂ emissions and is based on exergetic allocation. The input variables are thermodynamic CHP parameters. The allocation of CO₂ emissions assesses the shares for maintenance, production, biomass supply chains and unburned emissions of CH₄ and N₂0. The validation of the model is carried out by means of test bench analyses on an MAN D 26 common rail engine modified into a diesel cycle, a dual fuel cycle and an Otto cycle under different load conditions and compression rates. The engines are operated with biodiesel, rapeseed oil, palm oil, soybean oil, biomethane and wood gas. The CO₂ mitigation costs depend on the reference technologies. Furthermore, the unburned CH₄ emissions, the CO₂ emissions of the biofuel supply chains, the biofuel costs as well as the electrical efficiencies are significant influencing factors. The modeling scheme provides a scientific basis for the calculation and optimization of CO2 mitigation costs of biomass CHP systems for a wide variety of applications.