The mechanical properties of thermoset adhesive layers depend greatly on the cure conditions, such as cure temperature and duration. Ultrasonic measurements of the cure state of thermosets have been shown to be very sensitive to variations in these process conditions. In this paper a further parameter affecting the cure state of thermoset layers is considered. The thickness of an adhesive layer is shown to have a significant effect on the final properties of the thermoset. We suggest that the exothermic nature of the cure reaction is causing significant temperature gradients to occur within adhesives layers during cure leading to a variation in properties across the layer together with the possibility of decreased bondline strength. © 2004 American Institute of Physics