Monolayer adsorption of benzoic acid derivatives on activated oxide surfaces from solution and from the vapor phase are investigated with the view of modifying the work function of a conductive substrate. Para-substituted benzoic acids with a wide range of electrical dipoles allow adsorption to be followed by measuring the surface potential of the grafted substrates using the Kelvin probe technique. From the linear correlation between adsorbed molecular dipole and work function, it is possible to obtain the dipole contribution of the anchoring group. The vapor growth method has striking advantages over the monolayer growth from solution: it is extremely fast and avoids numerous problems related to solution processing such as intercalated solvent molecules in the dipole layer or chemical reactivity between the solvent and the oxide surface.