The phase behavior and hydrogen bonding in ternary polymer blends of phenolic resin, poly(ethylene oxide) (PEO), and poly(epsilon -caprolactone) (PCL) were investigated by using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). Although all three binary blends are respective miscible, there exists a closed immiscibility loop in the phase diagram due to the so-called "Delta chi" and "DeltaK" effects in this hydrogen-bonded ternary polymer system. The interassociation equilibrium constant based on the Painter-Coleman association model between phenolic resin and PEO can be indirectly calculated from the fraction of hydrogen-bonded carbonyl groups. Quantitative analyses show that the hydroxyl-ether interassociation is more favorable than the hydroxyl-carbonyl interassociation at room temperature. The interaction energy density value of B-PEO/PCL = -2.85 cal/cm(3) was obtained by comparing the theoretically predicted phase diagram with experimental data.