The use of ultrasonic spectroscopy for the non-destructive determination of the cohesive properties of the adhesive layer in a joint is described. It has been shown that measurements of the through thickness natural frequencies of the joint obtained using ultrasonic spectroscopy can be used to calculate the modulus and thickness of the adhesive layer. The results reported here indicate that the modulus may be determined to an accuracy of ±6%, and the thickness may be found as accurately as it can be measured independently. No other test is available which enables both these parameters to be determined after a joint is made, and it is anticipated that the test will be very valuable in ensuring that the process quality control during the joint manufacture has been satisfactory.