The passivating behaviour of self-assembled octadecanethiol monolayers on polycrystalline gold electrodes was tested by means of the cyclic voltammetry technique in the presence of three different redox probe molecules. The three compounds chosen for this purpose, ruthenium hexamine chloride [Ru(NH3)Cl-3], potassium ferrocyanide [K4Fe(CN)(6)] and benzoquinone, were selected on the basis of the differences in their heterogeneous electron transfer rate constants and their hydrophobic/hydrophilic character. Our studies show that passivating properties of octadecanethiol monolayers against these probes are different. The results obtained are discussed in terms of Tafel approach and monolayer integrity. It appears that the screening of self-assembled films with different probes is necessary in order to properly characterise the film integrity and passivating behaviour. Additionally, the interaction of octadecanethiol molecules with a polycrystalline gold surface was evaluated by means of potentiostatic pulse measurements.