The ultrasonic absorption of 1‐proline, 1‐serine, and 1‐arginine were measured in the frequency range of 3–90 MHz using an automatic recording ultrasonic spectrometer. Ultrasonic velocity was measured at 5 MHz using a single‐crystal interferometer of a variable path, and hypersonic velocity was measured by Brillouin scattering. To study the reaction, appropriate pH values were achieved by the addition of NaOH. Ultrasonic absorption as a function of pH value in all amino acids show a maximum, and so it is considered that the obtained excess absorptions are due to proton transfer reaction. The observed spectra of 1‐proline and 1‐serine have been analyzed in terms of single relaxation behavior, and the rate constants and the volume change have been obtained. Ultrasonic absorption of 1‐arginine show two relaxations due to α‐amino group and β‐guanidine group. In the latter one, the ultrasonic absorption α/f2was constant below 90 MHz, and then the relaxation frequency would be in the far region above the present experimental frequency. Therefore, the ultrasonic velocity at 5 MHz was compared with the hypersonic velocity.