For Gas Chromatography Chemical Ionization Mass Spectrometry (GC-CI-MS) method development, a standard lithium-ion battery (LIB) electrolyte was thermally aged at 95 °C for a faster generation of decomposition products. Interestingly, phosphorous containing aging products were preferably formed in the first hours. Furthermore, the organic containing aging products without phosphorous are formed after longer times. Since, these compounds are very similar in structure, molecular mass and consequently in their fragmentation during electron ionization (EI) experiments, chemical ionization was applied to identify the structure of the LIB electrolyte aging products. Herein, the results of the structure determination of organic LIB electrolyte aging products with GC-CI-MS are presented. Furthermore, formation of the identified species and relative concentrations over a time period of 22 days were determined by using an internal standard.