The phase behaviour and thermodynamics of poly(1,4-phenylene ether sulfone) (PES) and poly(ethylene oxide) (PEO)/alkali-metal salt complex blends were investigated by means of differential scanning calorimetry (DSC) and modulated DSC (MDSC). Experimental results show that the blend systems remain miscible after incorporating various alkali-metal salts: CF 3 SO 3 Li, CF 3 SO 3 Na and CF 3 SO 3 K. The cloud point temperature strongly depended on the Li (Na or K)/O ratio in the PES-PEO/alkali-metal salt complex blends. With increasing the Li + (Na + or K + )/O ratio, the phase diagram of the PES-PEO/alkali-metal salt complex blends tended to be symmetrical. When Li + /O = 0.02, the lower critical solution temperature (LCST) of the PES-PEO/CF 3 SO 3 Li complex blends was located at the 30/70 PES/PEO composition. The mixing enthalpy decreased in the PES-PEO/alkali-metal salt complex blends with increasing Li + (Na + or K + )/O ratio. The radius of ion has significant influence on the phase behaviour of PES/PEO blends. MDSC results showed that the change of heat capacity at the temperature of the binodal phase separation is similar to that of a melt transition in semi-crystalline polymers, which confirms the mechanism of binodal phase separation: nucleation and growth.