For paramagnetic ions with a long electron spin relaxation time, such as Mn2+, the measurement of the magnetic field dependence of the proton spin-lattice NMR relaxation time (referred to as nuclear magnetic relaxation dispersion) provides an indirect way to determine the number of ions, either free in solution or adsorbed onto the surface, in aqueous colloidal silica suspensions. At pH values higher than the pH of precipitation of the metal-hydroxide, the proton relaxation enhancement results from adsorbed ions only. By adding sufficient ions, saturation of the accessible silica surface was obtained and a maximum coverage of 0.43 Mn2+ ions per nm(2) was found. For Ni2+, a fast relaxing paramagnetic ion, the proton NMRD curves do not allow us to distinguish between free and adsorbed ions. It appears that in NiCl2 solutions the proton spin-lattice relaxation time substantially reflects the chemical exchange time of the protons complexed to Ni2+. Under these circumstances, one can evaluate the rate constants of the uncatalyzed and the acid-base catalyzed proton exchange in Ni2+ aquo-complexes from the pH dependence of the proton relaxation rate.