The role of solvation and structural equilibria in determining the ultrasonic absorption of aqueous solutions of bovine serum albumin has been investigated using chemical agents to denature the protein in solution. The agents used were methanol, ammonium sulphate, and guanidine hydrochloride. Two different techniques of measurement were used to cover the frequency range 200 kHz to 30 MHz. The results show that only guanidine hydrochloride produces a significant change in ultrasonic absorption even though the other agents were used at concentrations close to those which would produce protein precipitation. The conclusion is that in the above frequency range structural and solvation-related equilibria are unimportant and only perturbation of proton-transfer equilibria associated with the protein amino acid residues leads to the bulk of the ultrasonic absorption.