Experimental thermodynamic methods (adsorption isotherms and microcalorimetry) have been applied to the adsorption of &ggr;‐aminopropyl triethoxysilane and of more simple molecules (triethylsilanol., mono di and triethoxyalkylsilanes which have common reactive functions with the aminosilane molecule) on silica. It is shown that the amine function gives to the aminosilane molecule specific superficial properties: it catalyses (like in solution) the superficial hydrolysis of the ethoxy groups (as shown by the appearance of alcohol in the equilibrium solution) which allows subsequent grafting by siloxane bond with the surface. The influence of surface water on the siloxane bond formation is stressed for both the aminosilane and triethylsilanol molecules. Nevertheless, as shown by calorimetry and probe molecules (here phenol and pyridine) most NH2groups are also interacting with the surface. ©1996 American Institute of Physics.