Ultrasonic absorption spectra between 800 kHz and 2 GHz and sound velocities of aqueous solutions of calcium nitrate have been measured at different salt concentrations and temperatures as well as solutions of calcium chloride and magnesium nitrate at 25 degreesC. All solutions reveal a relaxation region with discrete relaxation time between 200 and 500 ps (nitrates, 25degrees) or at 100 ps (chloride, 25degrees). The amplitude of the relaxation term is substantially larger with the calcium nitrate system than with the other electrolyte solutions with idential salt concentration c. The relaxation time of Ca(NO3)(2) solutions is independent of concentration and the relaxation amplitude increases linearly with c, indicating a unimolecular reaction. The high-frequency ultrasonic relaxation is thus assumed to be due to the equilibrium between an outersphere ion complex and a contact ion pair.