High-pressure acoustic properties of poly(methyl methacrylate) (PMMA) was investigated up to 31.5 GPa by using a Fabry-Perot interferometer and a diamond anvil cell. Both backscattering and forward, symmetric scattering geometries were used to derive the pressure dependences of the longitudinal sound velocity, the refractive index, and the density over the investigated pressure range. These physical properties showed rapid increases upon compression up to similar to 5 GPa, above which they exhibited sluggish increases upon further increase. The crossover behavior at similar to 5 GPa was attributed to the change in the densification of PMMA caused by complete collapse of free volume in this polymeric material. (C) 2015 Elsevier B.V. All rights reserved.