Miscibility and morphology of poly(ether sulfone)/poly(vinyl pyrrolidone) (PES/PVP) blends containing a crystalline phytochemical called mangiferin were investigated using differential scanning calorimetry (DSC), Fourier transformed infrared spectroscopy (FTIR), and polarized optical microscopy (POM). The binary blends of PES/PVP were found to be completely miscible. However, FTIR experiments revealed no spectral shift that is attributable to the miscibility of the PES/PVP pair, although the occurrence of hydrogen-bonding interactions can be confirmed in binary blends of both PES/mangiferin and PVP/mangiferin. The addition of mangiferin to the PES/PVP blends resulted in liquid-liquid phase separation as well as liquid-solid phase transition. However, the liquid-liquid phase separation was observed only in a very small ternary composition range of the PES/PVP/mangiferin blends. With further increase of mangiferin concentration, crystallization occurred, leading to phase segregation between the isotropic liquid (PES/PVP) phase and the crystalline mangiferin. A ternary morphology phase diagram of the PES/PVP/mangiferin blends was established based on the evidence from DSC and POM experiments, which exhibited various coexistence regions including isotropic, liquid + liquid, liquid + crystal, and solid crystal regions.