This work details the formation of organic monolayers on Si(111) by electrochemical methods. We show that grafting of phenyl groups is possible by reduction of N-+(2)-Ar-X cations where the substituent X may be Pr, NO2, COOH, CN, CnH2n + 1 (n = 1, 4, 12). Characterizations show that the electrochemical process is self stopped after completion of the first monolayer whose structure is (2 x 1) close packed in the case X = Br and CH3 as observed by STM. The stability and passivating properties of films are also investigated.