The reactivity of colloidal particles is regulated by their surface properties. These properties affect the wettability, flocculation-dispersion characteristics, ion exchange, sorption capacities and transport of inorganic colloids. Most studies have focused on hydrophilic, charged particle surfaces, often neglecting the alterations in surface properties produced by the adsorption of natural organic matter, surfactants and other compounds. Adsorption of these substances can potentially render a surface substantially more hydrophobic. Nevertheless, comparatively little is known about changes in surface properties and reactivity of minerals upon sorption of hydrophobic organic compounds. In this study, the properties of four minerals (kaolinite, pyrophyllite, montmorillonite and Min-U-SilŪ) and two inorganic materials (amorphous aluminum hydroxide and amorphous silicon oxide) were compared before and after treatment with the common silylating agent trimethylchlorosilane (TMCS). The samples were characterized by measurements of total carbon, cation exchange capacity, particle size, specific surface area, electrophoretic mobility, contact angle, particle aggregation, x-ray diffraction and by diffuse reflectance infrared spectroscopy. For the layer silicates, surface coverage was limited to ≤2% trimethyl silane (TMSi). TMSi covered 7.5% of the Min-U-SilŪ surface and 33% of the amorphous silicon oxide. Treatment did not affect the structure of the minerals but lowered the cation exchange capacity, specific surface area and electrophoretic mobilities. Water contact angles increased between 18 to 114 degrees with treatment. While the apolar characteristic of the surfaces decreased minimally with treatment, the Lewis acid/base properties were substantially reduced and interfacial free energy shifted from positive to negative values indicating a more hydrophobic surface character. For all the samples except kaolinite, these changes affected the stability of the colloids in suspension depending upon solution pH. Even though the grafting of TMSi altered colloidal mineral surface properties and increased their hydrophobicity, these changes were not sufficient to predict colloid aggregation behavior.