The behaviour of plutonium on cooling from the delta-phase has still not been fully resolved. A number of studies by differential scanning calorimetry (DSC) and dilatometry on the delta ->gamma phase transformation have reported the unusual behaviour of this transformation and have proposed a range of very plausible explanations for the mechanism based on the conditions examined. The current study, which has been conducted over a much wider range of experimental conditions, shows new evidence indicating that the transformation is influenced by recovery processes that occur in the delta-phase. The transformation will adopt a different mechanism depending on the condition and thermal history of a specimen. The transformation may be measured by DSC, for specimens subjected to different thermal conditioning, as a barely visible undulation in the baseline, a series of well defined discrete peaks extending over a large temperature range or as this work demonstrates a single defined peak. This study also shows evidence that the plutonium phase transformations on cooling are unlikely to strictly follow the sequential transformations that mirror the phase transformations on heating. A mixed sequential and serial phase transformation route is proposed. Crown Copyright (c) 2013 Published by Elsevier B.V. All rights reserved.