Solid–liquid phase diagrams of binary crystalline blends of genistein with poly(ethylene oxide) (PEO) and poly(ethylene glycol) (PEG) were established experimentally and theoretically based on the combined Flory–Huggins free energy of liquid–liquid phase separation and the phase field free energy pertaining to crystal solidification. The liquidus lines obtained self-consistently were found to agree well with trends of depressed crystal melting transitions in genistein/PEO and genistein/PEG blends, exhibiting eutectic phase behavior. Of particular importance is the lowering of the eutectic temperature of the genistein/PEO blend by about 60 °C upon switching to the genistein/PEG system. The occurrence of interspecies hydrogen bonding between genistein molecules and both PEO and PEG chains, albeit weak, was noticed by Fourier transform infrared spectroscopy. The improved solubility of genistein in PEG can be attributed not only to lowering of the molecular weight of PEG utilized, but also to its terminal hydroxyl groups. This eutectic melting approach by PEG solvent is sufficiently effective in solubilizing genistein crystals that development of genistein-containing drugs might be feasible for injection and/or oral administration.