In order to obtain information about the structure of solutes and the ultrasonic characteristics in aqueous solutions of heterocyclic amines, ultrasonic absorption coefficients in the frequency range from 3 to 220 MHz have been measured in aqueous solutions of morpholine and piperidine at 25 degrees C. Only a single relaxational absorption has been observed in both solutions. The cause of the relaxation in their relatively dilute solutions (less than around 2 mol dm(-3)) has been attributed to a perturbation of an equilibrium associated with a hydrolysis of the amines, The rate constants have been determined from the hydroxide concentration dependence of the relaxation frequency. The standard volume change of the reaction has also been estimated from the maximum absorption per wavelength. The large standard volume change has been discussed in relation to the hydrated solvents near the reactant molecules. For the solution of morpholine, the measurements of the absorption and velocity have been also carried out in the concentrated solutions. The peak sound absorption concentration (PSAC) and the peak sound velocity concentration (PSVC) have been observed. However, it has been found that these characteristic ultrasonic properties are slightly different from those in 1,4-dioxane solution.