Knowledge of variations in the optical properties of liquids with pressure is required in ultrasonic‐light interaction studies. One parameter used to describe these variations is the piezooptic coefficient, defined as the change in refractive index produced by a unit change in the applied pressure. The relative adiabatic piezooptic coefficients of numerous pure liquids and alcohol‐water mixtures were determined from measurements made on light diffracted by pulsed ultrasonic waves. Absolute values of this parameter for the liquids tested were calculated from a consideration of experimental values previously determined and the results compared with the theoretical and empirical formulas presently used to compute this parameter. [Research sponsored by the Office of Naval Research.]