Ultrasonic absorption coefficients in the frequency range from 6.5 to 220 MHz have been measured in aqueous solutions of 3-ethoxy- and 3-butoxypropylamine as functions of their concentrations along with the sound velocity at 2.5 MHz and the density. In a 3-ethoxypropylamine solution, only one relaxational absorption has been observed up to 3.0 mol dm−3. The cause has been clarified to be due to a proton-transfer reaction of the amine from the concentration dependences of the relaxation frequency and the relaxational amplitude. The rate and thermodynamic parameters have been determined and reasonable values have been obtained as a diffusion-controlled reaction. In the solution of 3-butoxypropylamine, another relaxation process in addition to that due to the proton-transfer reaction has been observed in the concentrations of more than 0.5 mol dm−3. This relaxation process has been attributed to a perturbation associated with an aggregation reaction, 4A↔A4, from the concentration dependences of the relaxation frequency and the relaxational amplitude of the absorption. The rate and thermodynamic constants for the proton-transfer and aggregation reactions have been determined. It has been found from a comparison of the results for the two solutions and others reported previously that the introduction of ether oxygen into amine molecules has little influence on the proton-transfer reaction, but prevents the formation of aggregates due to the hydrophobic interaction. It has also been seen from the results of 3-butoxypropylamine solutions that the butoxyl group has a fairly high ability of hydrophobic interaction and that nonionized molecules form aggregates in the aqueous solution.