The acoustic properties of racemic S(+)-R(-) ibuprofen single crystals and amorphous ibuprofen were investigated by Brillouin light scattering. The temperature dependence of the longitudinal acoustic (LA) mode in crystalline ibuprofen could be explained by normal anharmonic lattice models. The LA mode frequency of amorphous ibuprofen exhibited a clear change at similar to 228 K at which glassy ibuprofen changes into supercooled-liquid state. The acoustic damping of the LA mode in both glassy and crystalline ibuprofen were similar. This indicated the molecular fluctuations remnant in the glassy ibuprofen are comparable to crystalline ibuprofen, which may be attributable to the strong hydrogen-bonded aggregates in both states. (C) 2011 Elsevier B.V. All rights reserved.