It was established that incomplete substitution of free silanols of fumed silica surface on (CH 3 ) 3 Si-groups leads to an increase in the ability of modified SiO 2 to adsorb Ba 2+ cations from an electrolyte solution. Silica with 48.3% of trimethylsilyl (TMS) groups adsorbs 1.8 mmol·g -1 of Ba 2+ cations from 0.01 M BaCl 2 solution, that in 3 times more than the adsorption of these cations by unmodified silica.It was found that the adsorption of cations by basic and modified silica is well described by the pseudosecond order Lagergren equation. The high adsorption activity of silica with chemisorbed TMS-groups is due to the formation of local regions with relatively large values of negative and positive electrostatic potentials in the vicinity of grafted TMS-groups.