In this work the discharge mechanism for heat treated electrolytic manganese dioxide (HT-EMD) in a primary lithium battery is investigated. In-situ and ex-situ synchrotron X-ray diffraction is utilized to study the structural evolution with respect to material utilization (Li x MnO2). At a 4.3 mA g−1 discharge rate, HT-EMD undergoes a simultaneous two-phase transformation and solid solution reaction for 0 < x ≲ 0.21 . Beyond this point both these processes continue in conjunction with a solid solution reaction of the lithiated phase (observed for x > 0.34) formed in the two phase process. At higher discharge rates (17.1 mA g−1), no evidence for the new lithiated phase is observed, suggesting the discharge mechanism is rate dependent. These findings represent a key step in understanding the mechanistic behaviour of HT-EMD in primary battery systems, which in turn will assist in optimizing battery performance characteristics and broadening possible electronic applications.