We have studied the formation of phenyl layers by electrochemical reduction of aryl diazonium salts (N-+(2)-Ph-R with R = Br and CH2Br) on atomically flat H-terminated Si(111) electrodes. The surface density, compactness and thickness of layers are investigated as functions of the electrolysis parameters used during the modification (applied potential, charge passed) and the end group, using Rutherford backscattering measurements, electrochemical capacitance-voltage curves, and AFM/STM observations. Results indicate that some surface polymerization occurs above a critical charge Q*, the value of which depends on both the potential and the R-group. Results are interpreted within the frame of a kinetic model. It is shown that dense and ordered monolayers are nevertheless obtainable by monitoring the charge Q close to Q* and by proper choice of the grafting potential. Kinetic Monte Carlo simulations and structural models of the interface are presented to discuss the formation of 2D domains in monolayers. The electronic properties of the modified interface are discussed briefly. (C) 2003 Elsevier B.V. All rights reserved.