Poly(ethylene oxide), PEO, is known to undergo a phase separation transition in aqueous solutions upon heating. This LCST transition is first investigated by high-sensitivity differential scanning calorimetry. Dependences of the transition temperature (T-t), enthalpy (Delta(t)h) and heat capacity increment (Delta(t)c(p)) on the polymer concentration are determined. The transition temperature and enthalpy of PEO decreased with increasing the polymer concentration and followed the Kirchhoff law. A substantial positive value of the transition heat capacity increment of PEO is detected suggesting that the transition is accompanied by an exposure of the polymer hydrophobic groups to water. Comparison of Delta(t)c(p) with the corresponding change in the accessible surface area of PEO reveals that the LCST transition of PEO may involve cooperative conformational changes of the type of helix-coil transition in PEO macromolecules. (C) 2015 Elsevier Ltd. All rights reserved.