Brillouin light scattering has been used to measure the sound velocities in polystyrene of two different molecular weights. The variation of sound velocity with temperature is related to volumetric expansion of the polymer. Using the thermal expansion data of Fox and Flory we have determined the Grüneisen number, γ, below the glass transition to be 4.4 and 4.3 for low‐ and high‐molecular‐weight polystyrenes, respectively, and 5.1 and 5.6 above the glass transition. These numbers compare reasonably well with the value of 3.9 determined by Wada et al. Our measured values are in good agreement with Barker's “bundle of chains” model.