The rate at which harmonics are developed during the propagation of an initially sinusoidal ultrasonic wave of finite amplitude depends on the nonlinearity of the compressibility of the medium. Thus, determinations of the harmonics as a function of propagation distance may be used to determine the parameterB/A(describing nonlinearity). A method which circumvents the effects of absorption in the medium is developed and used to determine the parameterB/Afor water andm‐xylene. This method utilizes optical determinations of the second harmonic component of a distorted 3.0 Mc ultrasonic wave at various distances and initial pressure amplitudes. The second harmonic was isolated by use of a calibrated acoustic filter plate. The values obtained for the nonlinearity parameter areB/A = 6.2±0.6for water andB/A = 9.6±1form‐xylene.