It has been evident for some time that the unusual structural behavior found in plutonium is related to the localization mechanism for the5felectrons in going from alpha to delta plutonium. The atomic volume expands by sharply defined steps at the alpha-to-beta, beta-to-gamma, and gamma-to-delta transition temperatures. Here we explain this behavior on the basis of a multistep5flocalization process involving spatially nonuniform localization. That is, we suggest that a disordered array of two types of plutonium on crystallographically equivalent fcc sites breaks the translational symmetry and provides an entropy generating mechanism that drives the transition to the delta structure. ©2000 American Institute of Physics.