The resistance to slow crack growth (SCG) was investigated in binary blends of high-density polyethylene (HDPE) and a model monodispersed ethylene-butene copolymer. The results were correlated with the morphological structure of the blends by a simple quantitative theory. The time to failure (t(f)) depended on crystallinity, crystal thickness, and most importantly the intrinsic resistance to SCG of the crystal network as determined by the density and branching of its tie molecule. t(f) was measured from 30 to 80 degrees C. At lower testing temperatures from 30 to 60 degrees C, the blend with the thinnest crystals had the longest t(f). However, at the higher temperature of 80 degrees C, the blend with the thinnest crystals became weaker relative to the blends with thicker crystals. The activation energy increased as the crystal thickness decreased.