This work presents new data on modifications to Si[111] surfaces by reduction of diazonium salts. It is shown that a monolayer thickness requires a careful control of the amount of radicals generated. Capacitance measurements performed under accumulation at n-type Si[111] modified surfaces indicate a small density of electronic states at the Si \ molecule interface. It is further demonstrated that analyzing capacitance data in terms of an effective dielectric constant may be used to characterize the layer structure. The influence of the molecule structure on the density of layers and the re-oxidation at modified surfaces are thus investigated. The mechanism of grafting is also discussed.