4-NO2 and 4-Br benzenediazonium salts have been electrochemically reduced on H-terminated Si(111) electrodes. Electrochemical measurements evidence that the reaction results in a robust modification of Si(111) surfaces. XPS shows that organic films are monolayer thick and that covalent =Si-Ar bonding occurs, with no oxide at the interface. In the case of the Br salt, quantitative RES measurements suggest that layers are (2x1) close-packed and assess their stability against several rinsing procedures including exposure to 40% HF. A mechanism of grafting is discussed.