This study based on in situ differential electrochemical mass spectrometry (DEMS) in model cells with Li(Ni, Co, Al)O2 and Li(Ni, Mn, Co)O2 cathodes highlights various influence parameters on CO2 evolution in lithium-ion batteries. The amount of CO2 formation is increased by high temperatures and cell voltages, while the addition of vinylene carbonate (VC) decreases it. Lithium carbonate present in the cathode causes increased CO2 formation in the initial cycles but has little influence during prolonged cycling. Two processes of CO2 formation with different kinetics could be distinguished. The amount of CO2 evolved depends strongly on the type of cathode active material.