Activities of water are reported for aqueous solutions of poly(ethylene glycol) (PEG) with number average molecular weights of 300, 400, 4000, and 6000 are measured at 35, 45, 55, and 65 degrees C and at various concentrations from 0 to 40 wt % of PEG using vapor-pressure osmometery. A cubic equation in terms of PEG concentrations is sufficient for correlation of activities. Using measured activities, the Flory-Huggins interaction parameters have been calculated. The effect of temperature, molecular weight of PEG and its concentration, and volume change upon mixing on interaction parameter have been considered. Ignoring the volume change of mixing can lead to an average error of 2.5% with a maximum error of 5.3%.