Morphological and thermal studies on high density polyethylene (HDPE) / glycerol ester of partially hydrogenated rosin (ester gum) blends reveal phase separation. Both thermal and dynamic-mechanical tests showed no shift of the HDPE glass transition temperature while the HDPE a, transition appeared. The presence of an additional transition was also noticed as the second component increased in blends; this was attributed to a rosin component. The peak ascribed to this transition became broader in blends enriched with oligomer; it moved toward lower temperatures due to the dissolution of low molecular weight HDPE. The melting temperature and crystallinity of HDPE varied slightly with the amount of amorphous oligomeric component in the blends.