A comprehensive Brillouin scattering experimental study of polypropylene glycol (PPG), average molecular weight of 1025, 2025, and 4000, has been carried out. The hypersonic velocity, attenuation coefficient, the modulus of elasticity, and the Landau–Placzek ratio have been obtained as a function of temperature for each polymer. The PPG 425 data reported earlier are also included for comparison. For all the quantities measured, only the Laudau–Placzek ratio (below ambient temperature) is molecular weight dependent; the hypersonic velocity, attenuation coefficient, and elastic modulus are found to be independent of molecular weight in the present 425–4000 weight range. The differences in the Landau–Placzek ratio between the different polymers are attributed to different degree of frozen‐in density fluctuations due to the difference in the medium’s viscosity; the mechanism of structural relaxation is not sufficient to explain the increase of the Landau–Placzek ratio of each polymer below ambient temperature. All polymers display the velocity dispersion and sound attenuation maximum. However, contrary to that observed in the ultrasonic frequency region, the maximum of the hypersonic attenuation coefficient appears to lie close to the beginning of the velocity dispersion curve and not its middle. The present results on the sound velocity and attenuation can be satisfactorily fit to a theory developed by Isakovich and Chaban, who have modeled the highly viscous liquid as a microinhomogeneous medium.