In this paper, accelerated rate calorimetry (ARC) and oven exposure, are used to investigate thermal runaway (TR) in lithium-ion cells. Previous work shows that lithium iron phosphate (LFP) cells have a lower risk of TR over other Li-ion chemistries. ARC is carried out on cells at various SOC to identify which decomposition reactions are contributing to the TR behaviour of a cell at different SOC. Results show, at SOC of 100% and 110%, the negative and positive electrode reactions are the main contributors to TR, while at lower SOC it is the negative electrode reaction that dominates. Cells at 100% SOC exposed to high temperatures during oven tests show, along with the ARC analysis, that the presence of the cathode and electrolyte reactions leads to an increase in the severity of a TR event for oven temperatures above 200 ∘ C . By comparing the heat generated in ARC and oven testing, it is shown that ARC does not fully capture the self-heating and TR safety hazard of a cell, unlike oven testing. This work gives new insight into the nature of the decomposition reactions and also provides an essential data set useful for model validation which is of importance to those studying LFP cells computationally.