Ultrasonic absorption measurements over a frequency range of 10-250 MHz are reported for some trivalent lanthanide nitrates, perchlorates, and chlorides in aqueous solution. Thermodynamic and kinetic parameters for the formation of MN032+ complexes of neodymium and gadolinium have been determined. The approximations and assumptions made in this calculation are discussed. Changes in relaxation frequency with cation across the lanthanide series are discussed in terms of changes in cation coordination number.