It has been well-known that aqueous solutions of poly(ethylene oxide) (or poly(ethylene glycol)) (PEO) are clear in a low temperature range but show clouding points due to phase separation behavior at rather high temperatures close to 100 degrees C slightly dependent on its average molar mass and concentration. We carried out extremely high frequency dielectric relaxation measurements up to 50 GHz over a temperature range from 10 to 70 degrees C to determine the temperature dependence of a hydration number per ethylene oxide monomer unit (m(EO)) in PEOs with the average molar masses higher than 3000. The obtained m(EO) was a constant value of ca. 4 in a temperature range lower than 30 degrees C and decreased rather gradually and approached ca. 2 at 70 degrees C. The temperature dependence of m(EO), hydration/dehydration behavior, confirmed in this study should be the essential reason for the phase separation behavior observed at high temperatures. The hydration/dehydration behavior observed in this study was compared with that reported previously using ultrasonic velocity measurement techniques.