Ultrasonic absorption coefficients in the frequency range from 0.8 to 220 MHz were measured in aqueous solution of propylamine in the presence of poly(vinylpyrrolidone) (PVP) with several polymerization degrees at 25 degreesC. In the solutions containing only the polymers, no relaxational absorption was found. However, a single Debye-type relaxational absorption was observed when propylamine coexists in solutions, and the cause of it was attributed to a perturbation of chemical equilibrium associated with a proton-transfer reaction. The protolysis and hydrolysis rate constants and the standard volume change of the reaction in the solutions with PVP were determined from the reactant concentration dependence of the relaxation frequency and the maximum absorption per wavelength. These results were compared with those in the solution with poly(vinyl alcohol) (PVA), and it was found that an interaction between propylamine and the polymer plays an important role in the proton-transfer reaction. The extent of the interaction between PVA and the reaction intermediate was proved to be smaller than that between PVP and the intermediate.