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 (LixMnO2). At a 4.3 mA g(-1) discharge rate, HT-EMD undergoes a simultaneous two-phase transformation and solid solution reaction for 0 < x less than or similar to 0.21. Beyond this point both these processes continue in conjunction with a solid solution reaction of the lithiated phase (observed for x > 034) 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. (C) 2014 Elsevier B.V. All rights reserved.