Quantitative nondestructive evaluation (QNDE) of the degradation of adhesive bonds remains one of the most challenging problems in QNDE. The objective of this research was to approach this problem by the detection of nonlinearity due to bond deterioration. The paper starts with experimental observations of the reflection of ultrasonic signals by adhesive bonds. The specimens for these tests had been subjected to cyclic loading which was expected to cause bond deterioration. Differences in the reflections could, however, be observed only if the adhesive bonds were subjected to static loads simultaneously with the ultrasonic testing. The higher the number of fatigue cycles, the lower the required load to display the differences in signals with unloaded cases. The second part of the paper presents a theoretical explanation of these ultrasonic measurements based on the postulate of nonlinear stress-strain behavior of the deteriorated bond. The ultrasonic tests provide the slope of the stress-strain curve and the results can therefore be used to determine the deviation of the stress-strain curve from linear behavior. For the higher numbers of fatigue cycles, this deviation, which is indicative of bond deterioration, starts at smaller load values.