Recent federal regulations concerning roadbed performance have motivated research in polymer modified asphalt binders. Earlier studies on these binders have shown that many of them are susceptible to gross phase separation when the binder is stored at high temperatures under quiescent conditions. This phase separation, in turn, is affected by the initial morphology and storage conditions. In this effort we investigate the effect of particle morphology on the high-temperature emulsion stability of a low-density polyethylene-modified asphalt binder, as well as the concomitant effects on the mechanical behavior. We show for unstabilized emulsions that the dominant phase separation mechanism shifts from coalescence to creaming at a critical particle radius of 4 mu m at 110 degrees C. However, stabilized emulsions showed no evidence of gross phase separation for up to 48 hours at 110 degrees C. Dramatically different morphologies were observed for the unstabilized and stabilized emulsions; unstabilized emulsions having teardrop shaped particles and stabilized emulsions having both spherical particles and long cylindrical domains. The mechanical behavior, including the high-temperature dynamic viscoelastic behavior and low-temperature fracture toughness, increased with polyolefin content, but was insensitive to the particle morphology.