Pressure and molecular-weight dependences of acoustic mode behaviors of polystyrene polymeric material were investigated by using Brillouin spectroscopy. The longitudinal, the transverse and the bulk sound velocities were measured over a wide pressure range from ambient pressure to more than 10 GPa for five polystyrene polymers whose molecular weight ranging from 3700 to 979200. The longitudinal and the bulk sound velocities displayed nearly the same pressure dependence for all polystyrenes indicating that the effective free volume is very similar in five polystyrene polymers despite the huge change in the molecular weight. The Poisson's ratio slightly increased with decreasing molecular weight. (C) 2017 Elsevier B.V. All rights reserved.