Self-assembled monolayers (SAMs) of octyltrimethoxysilane (OTMS) were deposited onto indium-tin oxide (ITO) electrode surfaces. Contact angle measurements evidenced the hydrophobicity and homogeneity of the functionalized surface despite the intrinsic surface roughness of the polycrystalline ITO electrodes. The electrochemical properties of the OTMS monolayers were quantitatively analyzed in terms of resistance, dielectric thickness, diffusion constant, and defect area by cyclic voltammetry and impedance spectroscopy. It has been demonstrated that the alkylsiloxane monolayer acts as a diffusion barrier for ions in the electrolyte. Furthermore, a significant suppression of the charge transfer at the interface was observed, demonstrating the passivation effect of the monolayers against electrochemistry. In addition, the effect of alkyl chain length on the electrochemical properties was briefly analyzed by using octadecyltrimethoxysilane (ODTMS). The defect area of this self-assembled monolayer was reduced to 0.2%.