Ultrasonic absorption coefficients in aqueous solutions of adenosine 5＇-triphosphate (ATP) were measured at: 25 degrees C as a function of concentration and pH in the frequency range from 0.8 to 220 MHz. An ultrasonic relaxational absorption with two relaxation frequencies was observed near pH 3. However, above pH 12, only a single relaxational absorption was detected at around 100 MHz, and this relaxation is the focus of this report. The relaxation frequency is independent of the concentration and pH at around pH 12, and the maximum absorption per wavelength increases linearly with concentration. From these results, the cause of the relaxation was attributed to a syn-anti glycosyl isomerization reaction of ATP. The race of the rotational motion in ATP was found to be smaller than that for ADP and greater than those for AMP and adenosine;These results are discussed in relation to the charge distribution and the effect of the size of the phosphate group of the nucleotides. To examine the reaction mechanism further, ultrasonic absorption measurements were carried out for aqueous solutions of adenosine 5＇-monophosphate (AMP), adenosine S-diphosphate (ADP), and ATP at 15, 20, and 25 degrees C above pH 12 from which activation enthalpies were determined. Furthermore, the detailed temperature dependence of the absorption in AMP was investigated at several concentrations in order to determine the standard Gibbs free energy, enthalpy, entropy, and volume changes of the reaction, revealing the thermodynamic properties of the syn-anti conformational isomerization process of the nucleotides.