Ethylene/methacrylic acid (E/MAA) ionomers exhibit a complex morphology - consisting of polyethylene crystals, amorphous polymer segments and ionic aggregates - as well as pronounced differences in mechanical properties compared with the E/MAA copolymers from which they are derived. Here, we illuminate the microstructural origins of the changes in one Such property - the yield stress - imparted to E/MAA by partial neutralization with sodium. The yield stress reflects contributions from both polyethylene crystal plasticity and incomplete mechanical relaxation of the ion-containing amorphous phase; the amorphous phase, in turn, consists of ion-rich aggregates and ion-poor domains, with widely separated relaxation rates. The inability of the amorphous material immediately surrounding the ionic aggregates to relax, except at extremely low strain rates, greatly increases the yield stress of the ionomers. Only a minor fraction of the E/MAA groups must be neutralized to create ion-rich aggregates, and thus to achieve the limiting yield stress behavior. The slow growth of thin polyethylene crystals also has a marked influence: as they form after quenching from the melt, these secondary crystals bridge the gaps between the locally-vitrified amorphous regions, leading to a large increase in yield stress., (C) 2008 Elsevier Ltd. All rights reserved.