Measurements of the ultrasonic absorption and velocity were made on aqueous solutions of ammonia up to pressures of 2030 kg/cm2. The results show that the absorption decreases and the relaxation frequency increases as the pressure is raised. The data are consistent with the assumptions of Eigen that the excess sound absorption is due to a chemical relaxation effect brought about by the lag in the shift in the equilibriumNH3⋅H2O⇌NH4++OH−, and that the rate of the reaction is diffusion controlled.