Solvent activity measurements have been performed on the system poly(ethylene glycol) 400 (PEG 400) + methanol at T = (298.15 to 328.15) K using the improved isopiestic method. The obtained solvent activity data were correlated with the original and modified Flory-Huggins models. It was found that for the studied system at any mass fraction, as the temperature is increased the solvent activity and Flory parameter are both decreased, indicating an increase of interaction between polymer and methanol as the temperature is increased. The density and speed of sound data were also measured at T = (288.15 to 328.15) K for the PEG 400 + methanol system. From the obtained density values, the excess molar volume data were calculated and fitted to the Redlich-Kister equation. The excess molar isentropic compression values calculated from the measured speed of sound and density data were also fitted to the Redlich-Kister equation.