The use of the Modified Reactive System Screening Tool (MRSST) to study the thermal stability of electrolytes for lithium ion batteries was demonstrated. Important data for the understanding of the thermal behavior of lithium ion batteries (vapor liquid equilibrium data, heat capacity, heats of vaporization, reaction rates, and heats of reaction) can be obtained with the MRSST. The technique also allows sampling of the system (gas phase or liquid phase) at any time. The thermal stability of EMC and EC was analyzed using the MRSST. The results indicated that EMC is thermally stable until 320°C, while the EC decomposes at 263°C generating gases such as CO2, O2, and H2. The effect of O2 on the thermal stability of EMC was also studied. It was found that EMC thermally decomposes in the presence of O2 between 220 and 235°C generating non-condensable gases such as CO2.