At 25 degrees C ultrasonic absorption coefficients for aqueous solutions of 1-pentylamine in heavy water (0.0434-2.5093 moi dm(-3)) and in light water (0.0242-0.3071 mol dm(-3)) have been measured as functions of frequency in the range of 0.8-220 MHz and solute concentration. The sound velocity and density have also;been investigated in these solutions. The measured spectra have been analyzed in terms of a Debye-type relaxational equation, and two kinds of relaxational acoustic absorptions have been found. The causes of the relaxations have been attributed to a proton/deuteron transfer and an association-dissociation process. The forward and backward reaction rates, the equilibrium constants, the standard volume change of the reactions, and the adiabatic compressibility have been determined from the ultrasonic parameters, and the isotope effects have been evaluated. The results are discussed with those of the previous investigations for other amines, i.e., 1-butylamine, 1-propylamine, and ethylamine.