Sum-frequency spectroscopy has been used to obtain the vibrational spectra of two molecules, potassium oleate and sodium octanoate, adsorbed at the iron-water interface. These two molecules reduce the friction of iron-steel contacts: the surface structures inferred from the vibrational spectra indicate how these molecules lubricate iron under aqueous solutions. Potassium oleate causes a reduction in the friction of iron surfaces at 10 mM; the strength and phase of the resonances in the vibrational spectra of oleate indicate that a bilayer was adsorbed at the iron surface at this concentation. Sodium octanoate reduces the friction of iron surfaces under anodic potentials. The vibrational spectra of adsorbed octanoate did not contain any resonances at any applied potential: this observation suggests that a completely disordered film was adsorbed at iron. The comparison between the coefficients of friction suggests that the more ordered films formed by oleate are more effective at lubricating than the disordered octanoate films.