Above pH 12, a single ultrasonic relaxation absorption was observed at 100 MHz, while two relaxational frequencies were observed near pH 7 in 5'-GMP aqueous solutions. The single relaxation observed above pH 12 was attributed to a perturbation of the syn-anti rotational isomerization around the glycosyl bond based on the concentration and pH dependence of the relaxation frequency and the maximum absorption per wavelength. The kinetic and thermodynamic parameters were determined for the isomerization reaction. The results for the isomerization around the glycosyl bond in 5'-GMP are compared to those reported on related systems, and from such a comparison the effects of structural changes on the dynamics of syn-anti isomerization can be deduced.