The phase transition properties of poly(ethylene oxide) (PEO), in aqueous solution, have been examined by differential scanning calorimetry (DSC) in the presence of NaCl (0-2 M). DSC studies show, for the first time, that the PEO phase transitions have an unusual line shape, the exact nature of which is dependent on NaCl concentration. The T-m (temperature of maximum apparent excess heat capacity) decreases with increasing [NaCl]. FT-IR experiments reveal that changes occur in the conformation of the C-O-C backbone of PEG, in the presence of 2 M NaCl, at the same temperature as the discontinuity in the apparent excess heat capacity function observed by DSC.