A material strained beyond its yield point typically suffers substantial irrecoverable deformation. Surprisingly, this is not the case for ethylene/methacrylic acid (E/MAA) copolymers and ionomers, for which significant permanent deformation does not result until the applied strain exceeds 50-150%, far beyond the yield strain of 5-10%. At room temperature, strain recovery is complete on the order of hours or days following the removal of the applied load. Interestingly, the onset of permanent deformation coincides with a broad maximum or shoulder in the plot of stress versus strain. Two-dimensional X-ray scattering studies of both initially isotropic samples and highly aligned blown films reveals that this "second yield shoulder," commonly observed in the stress-strain curves of ethylene/alpha-olefin copolymers, is fundamentally associated with polyethylene crystal fracture, resulting in fragments of reduced lateral extent. Connections formed between these crystalline fragments lock in the deformed conformations of the amorphous intercrystalline segments, preventing the specimen from retracting to its initial dimensions. Additional recovery is possible through heating; complete melting of the deformed specimens results in full recovery up to applied strains of 200%, beyond which strain-induced chain disentanglement begins. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1588-1598, 2009