The complex transmission coefficient for a plastic specimen immersed in water (or other suitable liquid) is determined through use of a phase meter and a calibrated attenuator. Heterodyning the received rf pulses directly to video signals with a 10‐Hz beat frequency makes possible (1) the use of a commercial low‐frequency phase meter, and (2) an increase in signal‐to‐noise ratio of approximately 20 dB [H. J. McSkimin and T. B. Bateman, J. Acoust. Soc. Amer.45, 852–858 (1969)]. By way of illustration, measurements for attenuation and phase velocity are listed for a number of specimens, including Plexiglas (acrylate and methacrylate resin), Epon adhesive, epoxy foam, and several composite structures. The frequency and temperature ranges covered were 0.4–1.1 MHz and 20°–60°C, respectively.