Rayleigh surface waves at ultrasonic frequency have been utilized to determine shear modulus variations in the vicinity of adhesive‐adherend interfaces. The measurement technique is that of acoustic critical‐angle reflectivity conducted at sequentially exposed surfaces of adhesive through the interfacial accommodation zone toward the bondline. Basic theory is presented for determining shear modulus from measured Rayleigh critical angles at adhesive surfaces. Laboratory ultrasonic Rayleigh wave apparatus and its evaluation are described. Experimental procedures for sample preparation and measurement are discussed. Results of Rayleigh critical‐angle reflectivity measurements on two different types of adhesives display a decided gradient in shear modulus through the interfacial region. Reference is made to other applications of the ultrasonic Rayleigh wave approach in studying the shear properties near the bondline of adhesive materials.