The electrochemical behavior of the gold/electrolyte interface in aqueous 1 M NaNO3 solutions in the presence of an organic monolayer of n-decanethiol (CH3(CH2)9S) is studied by electrochemical impedance spectroscopy in the frequency range of 10–105 Hz and also by cyclic voltammetry. It is experimentally shown that in the potential interval from 0 to −0.5 V (vs. SCE), the dense monolayer film decreases the measured current density approximately 40-fold. The measured capacitance falls down to 1–2 μF/cm2. Based on the analysis of impedance characteristics acquired with the use of empirical equivalent circuits comprising ideal and nonideal analogues of electric circuits, the tentative estimates of the thickness of organic monolayers formed on Au electrodes with various roughness factors are obtained. Using the complex nonlinear regression (CNLS) method and a model of microarray electrode, the porous structure of adsorbed monolayers is revealed and the transition frequency of interfaces under study is determined. The degree of inhibition of the electron transfer across the Au/n-decanethiol/solution interface is determined by comparing the rate constants for the Ru[(NH3)6]3+/2+ redox process on clean and modified electrodes. The acquired results are compared with available literature data.