The growth of covalently bonded nitrophenyl layers on atomically smooth boron-doped single-crystalline diamond surfaces is characterized using cyclic voltammetric attachment and constant-potential grafting by electrochemical reduction of aryl diazonium salts. We apply atomic force microscopy (AFM) in contact mode to remove phenyl layers and measure phenyl layer thicknesses by oscillatory AFM. Angle-resolved X-ray photoelectron spectroscopy is applied to reveal the bonding arrangement of phenyl molecules, and transient current measurements during the grafting are used to investigate the dynamics of chemical bonding. Nitrophenyl groups at an initial stage of attachment grow three-dimensional (3D), forming layers of varying heights and densities. Layer thicknesses of up to 80 angstrom are detected for cyclic voltammetry attachment after five cycles, whereas the layer becomes denser and only about 25 angstrom thick in the case of constant-potential attachment. No monomolecular closed layer can be detected. The data are discussed taking into account established growth models. Redox systems such as Fe(CN)(6)(3-/4-) and Ru(NH3)(6)(2+/3+) are used to probe the electrochemical barrier properties of nitrophenyl groups grafted onto diamond.