To investigate the solute-solvent interaction between bile salts and water molecules [1, 2] ultrasonic absorption and velocity of aqueous solutions of ▪ Fig. 1. Sound velocity vs. frequency at c = 0.08, •; c = 0.34, ○; c = 0.47, ▴; and c = 0.72, ▵, mol/1. ▪ Fig. 2. Ultrasonic absorption vs. frequency at c = 0.08, •; c = 0.34, ○; c = 0.47, ▴; and c = 0.72, ▵; mol/1. sodium deoxycholate have been measured at 20 °C as a function of frequency (5–300 MHz) and concentration. Experimental results are in agreement with previous literature data [3]. Measurements of density and viscosity have been also performed in order to evaluate the absorption values derived from classical equation [4]. Results can be summarized as follows: 1. velocity dispersion is present at the higher concentrations. Its magnitude slightly increases with composition (Fig. 1); 2. large absorption excesses are observed in concentrated solutions. They decrease with concentration (Fig. 2). A comparison of the experimental losses with the ‘classical’ (viscosity) ones indicated that shear viscosity relaxation processes are present in highly concentrated solutions. Analysis of the absorption spectra also indicates the presence of structural relaxation phenomena [5] due to monomer-micelle and micelle-micelle equilibria. It appears possible to separate and evaluate the contributions from the several types of relaxation processes. They appear to be consistent with the observed magnitudes of the velocity dispersion.