A theoretical and experimental study is described for the optical birefringence associated with the propagation of a plane shear wave in a viscoelastic medium. The shear wave propagation is characterized by a complex propagation constant from which the complex coefficient of shear viscosity of the medium may be derived. The optical birefringence is related to the mechanical action of the shear wave by a complex mechano‐optic coefficient. In the experiment the birefringence due to the shear wave is analyzed by transmitting circularly polarized light through the material at right angles to the direction of shear wave propagation and to the direction of particle displacement. The modified, circularly polarized light is then analyzed by a plane analyzer and a photomultiplier detector. The mechanical and mechano‐optic constants for the medium are determined for an aqueous milling yellow solution and for a polystyrene solution and are compared with independent determinations of these same factors from other experiments.