This work establishes the kinetics and reaction mechanism for the gamma -> delta phase transformation in plutonium under non-isothermal conditions and examines the phase transformation parameters under conditions that have not previously been considered. The thermal history and condition of a specimen will have a significant effect on the reaction parameters of a solid phase transformation. The current work examines the influence of the specimen condition and more specifically the impact of a previous phase transformation on the gamma -> delta phase transformation parameters and kinetics. The gamma -> delta phase transformation parameters and activation energy profile were determined both immediately following the previous phase transformations and following an isothermal hold in the gamma-phase (50 h at 260 degrees C) to allow the specimens to recover from the impact of the previous phase transformations. A reaction mechanism for the gamma -> delta phase transformation is proposed based on the kinetic and mechanistic analysis. This transformation mechanism describes how the difference in structure and physical properties of the parent gamma-phase affects the nucleation and growth of the delta-phase. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.