A technique was developed for the determination of the primary hydration numbers of cations in aqueous solution. The method was devised to be applied in particular to the study of those ions for which the average lifetime of water molecules in the primary hydration sphere is less than ∼10-4 sec.The new technique is based on the kinetics of exchange of water molecules with hydrated manganous ions in solution and makes use of the relationship between this exchange and the proton nuclear magnetic resonance relaxation time T2 for such solutions. This relaxation time was found to be dependent on the nature and concentrations of added diamagnetic salts in a manner which could be related to the primary hydration numbers of the added cations. An empirical relationship was derived and tested as a function of concentration, temperature, anion and for cations of known primary hydration number, and the relationship was found to be satisfactory in all cases. The precision of the technique is estimated to be ±0.2 of a hydration unit. The empirical relationship was derived and tested with the ions Al3+, Be2+, H+, and NH4+. Ions to which the technique was subsequently applied included Mg2+, Ca2+, Sr2+, Ba2+, Zn2+, Cd2+, Hg2+, and Pb2+. The rather low numbers found for these ions are interpreted as a reflection of the role of the solvent structure of water in the determination of the structures of hydrated ions.