Isotherms of adsorption of Ni(II) on silica from 0.01 M NaClO4 are almost linear with a log-log slope of 0.95, and the percentage of uptake at a given pH is insensitive to the initial Ni concentration. At higher ionic strengths (>/=0.1 mol dm-3 ) two kinds of behavior are observed. For low Ni concentrations the adsorption is approximately equal to that observed at lower ionic strengths. For higher Ni concentrations (but still much lower than the concentration of surface sites) the Ni adsorption from 0.1 and 0.3 mol dm-3 NaClO4 is lower than the adsorption from 0.01 M NaClO4 by a factor of 2 (pH 9) to 3 (pH 8). When water is replaced by a mixed solvent (e.g., 2% aqueous DMSO, THF, methanol, glycerol) the Ni adsorption isotherms obtained in the presence of 0.1 mol dm-3 NaClO4 are linear with a log-log slope of 0.95. These adsorption isotherms are rather insensitive to the nature and concentration of the organic cosolvent. It is impossible to explain the Ni adsorption curves over the entire studied range of initial concentrations and ionic strengths in terms of surface complexation model, unless the Boltzmann factor is properly corrected.