The surface properties of organosilane-modified smectite-type clays were examined by various physico-chemical techniques, including thermal gravimetric analysis (TGA), surface area measurements from N2 adsorption/desorption experiments (BET method), Fourier transform infrared spectroscopy (FTIR), chemical stability measurements in aqueous medium, and elemental analysis. The organically modified clays derived from the reaction of two natural Cameroonian smectites and one standard bentonite sample (for comparison purposes) with γ-aminopropyltriethoxysilane and 3-mercaptopropyltrimethoxysilane. Efficiency of the grafting process was demonstrated by comparing the physico-chemical characteristics of the clays before and after reaction with the organosilanes, which resulted in decreased porosity and decreased amounts of surface hydroxyl groups concomitantly to the presence of aminopropyl or mercaptopropyl groups attached to the aluminosilicate structure. The amine-bearing materials were subject to rather low chemical stability in neutral aqueous medium, which was improved with decreasing pH, while the thiol-containing ones were found to be stable over a wide pH range (2–10). Possible use of these materials as adsorbents for mercury(II) species was discussed and some preliminary experiments based on carbon paste electrodes modified with such grafted clays highlighted their interest for electroanalytical applications.