In the first part of this publication, we describe details of the used DEMS setup and the data processing based on a multiple concentration determination (MCD) algorithm. In the second part, we discuss results of the DEMS analysis for sodium manganese nickel oxide (NaxMn3/4i1/4O2, abbreviated as NaMNO) as working electrode and Na metal as counter electrode. The gas formation is compared for two different electrolyte formulations with 2G and PC as solvents, with NaPF6 as the conductive salt, and for two different upper cut-off potentials. In case of 2G, CO2, methane and dimethyl ether are found as decomposition products when exceeding redox potentials of 3.8 V vs Na + /Na.