An optical heterodyne technique has been used to make new measurements of the ultrasonic properties of aqueous solutions of biomolecules, dimethyl sulfoxide, and 1,4‐butanediol. A LiNbO3transducer provided acoustic excitation from 0.2 to 1.5 GHz. Acoustic velocity and attenuation were measured using angularly resolved detection of Bragg‐scattered light from the liquid sample. A thermodynamic theory based on the heat of mixing was used to explain the concentration dependence of the liquid mixture properties. This paper contributes new data at high frequencies, a calculation of measurement sensitivity for this technique, and a complete treatment of a little known thermodynamic theory that describes these results.