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Differential electrochemical mass spectrometry (DEMS) is a powerful operando method for analyzing side reactions in batteries. We describe our DEMS setup highlighting the relevance for implementing a reference electrode. Although the method provides valuable information, the correct assignment of the DEMS signals to types of gases and quantifying the amounts released can be challenging. A frequent limitation is that gas concentrations are calculated from single m/z ratios. This has the drawback of overlooking unexpected gases which can cause misinterpretation of the signal intensities, or even attributing to gases which are not actually formed. We present a multiple concentration determination (MCD) algorithm that uses the full MS‐spectra, which allows a more reliable determination of the gas release. We demonstrate this approach for Na‐ion half‐cells with P2‐Na0.67Mn3/4Ni1/4O2 (NaMNO) as cathode active material (CAM). Studying the gassing behavior for two electrolyte formulations (1 M NaPF6 in propylene carbonate (PC) and in diglyme (2G)). Against the general belief that glymes lead to more gassing at high potentials, we find that gas evolution for PC electrolytes is larger compared to 2G electrolytes. Dimethyl ether is found to be a decomposition product of 2G. Pressure change measurements are used to independently validate the gas quantification. [ABSTRACT FROM AUTHOR]