The study of the interactions between an organosiloxane and a fumed silica was undertaken in order to improve our knowledge of the reinforcement of silicon rubbers by silica. IR spectroscopy, gravimetry, and chromatography techniques were used to characterize the chemical interactions taking place above 150°C between octamethylcyclotetrasiloxane and a fumed silica which yield silicas modified by grafted siloxanes. Reaction mechanisms were proposed for the interpretation of the grafting processes. They were checked by the study of the reaction of water with octamethylcyclotetrasiloxane. The results obtained suggest that the reactions proceed through an attack of the siloxane by the surface free silanol groups of silica, causing an opening of the siloxane cycle and the grafting of the subsequent material on the surface of the filler. It was shown that the siloxane-modified silicas exhibit a high thermal stability.