It is well known that reducing the crystal thickness of polyethylene, by copolymerization with an alpha-olefin, decreases the yield stress. By contrast, incorporation of methacrylic acid (MAA) - also a noncrystallizable comonomer - results in a marked increase of the yield stress at room temperature at typical strain rates. We show that, in addition to crystal plasticity, one must consider the active mechanical relaxations to understand this phenomenon. For ethylene-methacrylic acid copolymers, the alpha and beta relaxations are important over the range of conditions probed in this study. and the increase in the beta relaxation (glass transition) temperature with MAA content is identified as the source of this peculiar behavior. The yield stress of these materials is adequately described by a model combining thermal nucleation of dislocations in the crystals with a Ree-Eyring dependence for yielding in the amorphous phase, all with physically reasonable parameter values. Yield stress master curves may be created from data taken at various temperatures and strain rates, and are presented herein for low-density polyethylene and five ethylene methacrylic acid copolymers of varying MAA content. (C) 2008 Elsevier Ltd. All rights reserved.