Ultrasonic absorption coefficients in aqueous solutions of adenosine 5＇-monophosphate and adenosine 5＇-diphosphate were measured at 25 degrees C as a function of the concentration and pH in the frequency range from 0.8 to 220 MHz. At pH near 5, there existed two relaxational absorptions. One is observed at the frequency range <10 MHz with a large amplitude of the ultrasonic relaxation and a relaxation frequency which is strongly dependent on both the nucleotide concentration and pH. The source of the relaxation was attributed to a perturbation of the chemical equilibrium associated with the proton transfer reaction : N-1-R-PO42- + +HN1-R-PO4H- half arrow right over half arrow left 2N(1)-R-PO4H-. The forward and reverse rate constants were determined from the concentration dependence of the reactants and the solution pH. The forward rate constant obtained was similar to that previously reported. When the solution pH was increased to >11, the relaxation associated with the proton transfer reaction disappeared. A second relaxation is found at around 100 MHz, and its amplitude is smaller than that due to the proton transfer reaction. This relaxation is barely observed at neutral pH, but it is clearly distinguishable at high pH because the absorption associated with the proton transfer reaction is no longer observable. The relaxation frequency of the second relaxation is independent of nucleotide concentration and the solution pH, and the maximum absorption per wavelength increases linearly with concentration. It was concluded that the source of this relaxation is an isomerization process, probably the syn-anti interconversion of nucleotides. The value of the relaxation frequency in aqueous solutions of ADP was greater than that in AMP solutions, which in turn is greater than that for adenosine. The results are discussed in relation to nucleotide molecular structures and interactions.