Solvent activity measurements have been performed on the system poly(ethylene glycol) 400 (PEG400) + ethanol at T = (298.15 to 328.15) K using an improved isopiestic method. The solvent activity data were correlated with the original and modified Flory-Huggins models. The density, speed of sound, and viscosity data were also measured at T = (288.15 to 328.15) K for the PEG400 + ethanol system. From these experimental data, the excess molar volume, V-E, excess molar isentropic compression, K-s.m(E), and viscosity deviation, Delta eta, have been determined for each composition and fitted to the Redlich-Kister equation.