Sound velocity measurement is a widely used method for determining the compressibility of liquids, solutions, solutes, and macromolecules(1-9) and for the determination of a wide range of thermodynamic parameters.(10) Hitherto, the compressibility determined from sound velocity measurement has been interpreted as being adiabatic, and thermal scattering has been neglected. It is shown here that thermal scattering can be very significant and may be calculated by scattering theory. However, results indicate that the very large number density of scatterers in solutions, even at low mole fraction or volume fraction, causes additional scattering effects which are not yet included in scattering theory. It has previously been suggested that the heat flow associated with thermal scattering may produce a significant departure from adiabatic conditions.(9) However, it is shown that propagation remains adiabatic in the presence of thermal scattering, under the usual experimental conditions.