The thermal stability of lithium ion batteries was studied by means of Accelerating Rate Calorimetry in Heat-Wait-Search operation on both electrode and cell level. Fresh and aged samples were investigated depending on the state-of-charge (SoC) of a 5 Ah pouch cell comprising mesocarbon microbeads and LiNi0.4Co0.2Mn0.4O2 as the anode and cathode materials. 1 M LiPF6 in EC:DEC 3:7 (by weight) containing 2 wt% VC and 0.5 wt% LiBOB was chosen as the electrolyte. Measurements on the electrode level revealed a higher self-heating rate (SHR) of the cathode compared to the anode for all SoC and state-of-health (SoH) combinations in the temperature range where a self-sustaining decomposition reaction could be detected. A lower SoC showed a lower SHR of the electrode/electrolyte mixture with no reaction detected on the anode side <= 50% cell SoC. Cyclic aging led to a decrease in thermal stability of the cathode at lower SoC values with no significant influence on the anode implying a larger safety threat on the cell level. Avrami-Erofeev and autocatalytic reaction models were used to quantify the influences of SoC and SoH on reaction kinetics. Full cell measurements confirmed the observations at a higher SHR. (C) The Author(s) 2018. Published by ECS.