Some liquids and solutions become doubly refracting and behave as uniaxial crystals when traversed by acoustic waves of high frequency. In many aspects the induced birefringence is analogous to birefringence due to flow. Theories for this behavior are presented and discussed in detail. For liquids and solutions of flexible macromolecules the birefringence is proportional to the square root of the acoustic intensity and to the frequency of the wave, but for colloidal solutions it varies directly as the intensity and is independent of frequency. It is shown that from measurements of the birefringence, values of relaxation times and diffusion constants can be obtained. The method has a number of advantages over flow birefringence studies and in common with the latter does not depend on the possession of a permanent dipole moment by the molecules.