The adsorption of triethylsilanol (TES) on silica from a heptane solution has been studied in situ by microcalorimetry with a special attention to the water content of the adsorbent. In a first step, controlled rate transformation analysis, which allows one to distinguish between physisorbed and chemisorbed water, is used to prepare silica samples with a known surface concentration of water. Adsorption isotherms and displacement enthalpies (of the solvent by the solute) are then determined. A decrease of both affinity and displacement enthalpy is observed when the water surface concentration on silica is decreased. Moreover, irreversibility of adsorption (attributed to the formation of siloxane bonds) is observed only when physisorbed water is present : the siloxane bond formation seems to be catalyzed by the surface water.