Monolayers of n-dodecanethiol molecules on surfaces of polycrystalline nickel electrochemically pretreated are obtained by immersion either in pure n-dodecanethiol or in a solution of the mercaptan in absolute ethanol. The influence of the electrochemical pretreatment on the quality of the organic layer has been studied by contact angle measurements, electrochemistry, and X-ray photoelectron spectroscopy. It is found that n-dodecanethiol monolayers directly adsorbed on nickel surfaces without electrochemical pretreatment are of poor quality and exhibit a significant proportion of the thiols oxidized into sulfonates and sulfinates. On the contrary, the electrochemical pretreatment leads to monolayers with significantly improved physicochemical properties such as better chemical stability, resistance to electrochemical oxidation, and molecular order.