Newly developed hyperbranched polyols (HBPs) possess a compact, highly branched, three-dimensional (3D) structure, which has a high density of functional end groups and inherently low viscosity. The combination of these two properties, low viscosity and high reactivity, makes HBP polymers attractive candidates for reactive polymer blending. The HBP additives are able to behave as lubricants during processing and as self-compatibilizing toughening agents in the final blend formulation. In this work, we have studied a series of blends of hyperbranched polyester with high molecular weight polystyrenes. The processability and compatibility in the blends were investigated as a function of volume fraction of the HBP added and reactivity of the matrix phase. We find, through processing and rheological studies, that HBPs are extremely effective processing aids. A significant drop in the blend viscosity occurs immediately on addition of HBP, even at levels as low as 2 vol.%. Characterization including microscopy, thermal analysis and Fourier transform infrared spectroscopy (FTIR) indicates that the HBP forms immiscible blends with polystyrene and styrene maleic anhydride (SMA) copolymers. A significant degree of compatibilization occurs in the reactive systems, as evidenced by shifts in the thermal transitions of the HBP phase, and by morphology refinement observed by electron microscopy. The degree of compatibilization in the blends was found to increase with SMA reactivity. (C) 2000 Elsevier Science Ltd. All rights reserved.