The mechanism of the reaction between a silica sample coming from acid treatment of sepiolite (denominated Silsep) and an organosilane, namely bis(triethoxysilylpropyl)tetrasulfane (TESPT), has been evaluated by solid state NMR spectroscopy, being compared with the silanization reaction of a commercial silica. The effect of the silane concentration and temperature on the course of the reaction was considered. Experimental results indicate that the silanization reaction is more effective in the case of Silsep, favoring both the reaction of silane molecules with the filler surface and the reaction between neighboring silane molecules. This different behavior is attributed to structural factors, moisture, and number of acid centers on silica surface. Environmental scanning electron microscopy (ESEM) was used to deposit micrometric water drops on the surface of these samples and to evaluate the proportion and distribution of the organophylization process. (c) 2006 Elsevier Inc. All rights reserved.