We propose a molecular thermodynamic framework to describe liquid-liquid equilibria of hyperbranched polymer solutions. The model is based on the lattice cluster theory and the hydrogen-bonding model. We examine phase behaviors of hyperbranched polymer solutions in the effect of a branched structure and hydrogen bonding formations among endgroups of hyperbranched polymer and solvent molecules. The solvent-solvent hydrogen bonding dominates phase behaviors of hyperbranched polymer/water systems. The endgroups of hyperbranched polymers also play a great role in determining phase separation of highly branched polymer structure. (C) 2001 American Institute of Physics.