Polymer-modified binders are widely used for pavement construction. Among different polymers used to modify bitumen, polyolefins have shown to impart beneficial effects on the bitumen, especially to prevent some of the pavement shortcomings at high temperatures, such as rutting. The phase separation problems of these polymers from the bitumen may affect the final properties of the pavement. The determination of the polymer phase destabilization mechanism and rate are crucial to defer or delay the polymer separation from the matrix. With this aim, several HDPE-modified binders were manufactured by using a high-shear mixing device. The binders were stored in tubes at temperatures as high as 180 degrees C. During the storage, the unstable polymer phase creamed up to the surface. The destabilization process was followed by means of optical microscopy observations through time, as the separation process proceeded. The segregation of the polymer phase is shown to be preceded by a coalescence mechanism. Enhanced mechanical properties have been obtained by HDPE modification. However, such binders may not be efficient for the pavement application, since phase separation may occur during the application of the binder on the road. Functionalized reactive polymers are envisaged to hinder the separation problems in bituminous binders. (c) 2006 Wiley Periodicals, Inc.