We report the study of behenic acid Langmuir monolayers spread over chloride salts solutions of cadmium, lead, magnesium, or manganese. These monolayers were investigated by means of surface pressure -area isotherms and grazing incidence X-ray diffraction (GIXD). The effect of the concentration of cations in the subphase on the structure of the monolayers was probed at room temperature and for three different subphase pHs (5.5, 7.5, 10.5). A threshold in subphase concentration is detected for the formation of a superlattice structure corresponding to an inorganic organized layer in addition to the ordered behenic acid monolayer. This threshold is shown to strongly depend on the cation and the subphase pH. Above the threshold, the superstructure is independent of both the cation concentration and the pH. Below the threshold, the ions are disordered but induce a condensing effect on the fatty acid molecules, which is more or less pronounced depending on the ions. Moreover, the combination of isotherms and GIXD allows us to show that the existence of superstructures can be predicted from the shape of the isotherms. Indeed, a good agreement is obtained between the thresholds determined by the two experimental techniques.