The acoustic properties and their pressure dependence have been determined to 18.5 GPa for nearly amorphous poly(chlorotrifluoroethylene-co-vinylidene fluoride) (Kel-F 800) using high-pressure Brillouin scattering. At all measured pressures, both longitudinal and transverse acoustic modes were observed allowing for calculation of the pressure-volume isotherm for this predominantly amorphous material. Analysis of the room temperature isotherm using semi-empirical equation of state fitting forms to 5.5 GPa yielded a zero-pressure bulk modulus, Ko, and pressure derivative, Ko′, of 2.8 GPa and 30.6, respectively, which are consistent with the results from dilatometry measurements at very low pressures. Furthermore, the C11 and C12 elastic moduli for the isotropic polymer were determined at each pressure interval and, subsequently, examined to provide the pressure dependence of the bulk, shear, and Young’s moduli. These results are discussed in relation to polymer mechanics at pressures far exceeding those of previous, static compression experiments.