The silylating agent 3-trimethoxysilyl-1-propanethiol was immobilized on silica gel. Sulphur analysis showed 0.98 mmol of thiol groups per gram of support. These groups chemisorbed divalent cations : nickel, cobalt, copper, and mercury in the pH range from 1 to 6. The adsorption is more significant for mercury in relation to the other cations in the pH range studied. The primary amines, methylamine, ethylamine, n-butylamine, and benzylamine, were adsorbed on silica-thiol surface and on silica-thiol surface with mercury adsorbed. The values of adsorption constants were calculated using Langmuir isotherms data. Using these values, equilibrium constants and Delta G values for each adsorption system were also obtained. When mercury is present on the silica surface, the adsorption of n-butylamine and benzylamine is significatively enhanced in relation to the other amines. This behavior suggests that the degree of adsorption of the studied amines is intimately related to their dissociation constants.