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The quarter-wavelength (/spl lambda//4) acoustic matching layer is a vital component in medical ultrasonic transducers, which can compensate for the large acoustic impedance mismatch between the piezoelectric material and the human body. At high frequencies (/spl sim/100 MHz), the /spl lambda//4 matching layers become extremely thin, and the characterization of their properties becomes very challenging. We report a method to measure the phase velocity and attenuation of ultra-thin layers using the /spl lambda//4 matching principle, in which the acoustic impedance of the thin layer is between the substrate and water. The method has been successfully used to characterize epoxy films on glass substrate. The experimental results show good agreement in the phase-velocity measurement between our proposed method and the conventional ultrasonic spectroscopy method, but the attenuation measurement is sensitive to the properties of the substrate and water medium as well as the alignment of the sample.