Burst-continuous low-frequency ultrasound of 66 kHz frequency was used to examine the solid-propellant-to-liner bond as it undergoes failure due to constant-load application. Specimens with failure times as long as one week were studied. Piezoelectric-disk transducers of original design were used in a straight-through-transmission method. They were bolted to the sides of a circular-sandwich specimen; the solid propellant was bonded with flexible liner to the rubber insulation on one side and to an aluminum plate on the other. Failures in each case occurred in the flexible bond. Burst-continuous waves were sent through the specimen and the initial amplitude of the transmitted pulse was measured. The Fourier components of the transmitted pulse were obtained with an ultrasonic-spectrum analyzer and recorded on Polaroid film. In all cases tested, as the bond was failing, there was an increase in attenuation of the transmitted pulse and a shift in the characteristic “sonic” spectrum display. The results point the way for application of “sonic” spectroscopy to materials research.