A series of hyperbranched polyborates (HB) terminated with phenol hydroxyl (HBp) and boric acid hydroxyl (HBb) functional groups, respectively, were blended with phenolic resins (PR). The miscibility and specific interaction behaviors of the HB and PR were investigated by differential scanning calorimetry (DSC) and Fourier Transform infrared spectroscopy (FTIR). Cure behaviors and thermal properties of PR containing different content of HB were also studied. In addition, thermogravimetry analysis (TGA) exhibited apparent increase in thermal stability at both the initial stages and residue weight at 800 C under nitrogen atmosphere, especially, for the HBb10/90 system that has a temperature of 458 degrees C at 5% mass loss and a residue weight of 75.4%. Moreover, scanning electron microscope (SEM) revealed that the initially homogeneous phase separated into two-phase structure after the cure process, which offers a route to achieve new properties combinations that are desirable for high performance polymers. (C) 2008 Wiley Periodicals, Inc.