The influence of the divalent cations cadmium. lead and calcium on the properties of octadecanoic acid monolayers at the air-water interface is studied by infrared reflection-absorption spectrometry. The investigation is carried out for different cation concentrations and pH values of the subphase. It is shown for pH 6 that 1 mM Cd2+ or 1 mM Pb2+ leads to the formation of highly ordered structures even for large areas/molecule, while the ordering effect of Ca2+ at the same pH value is smaller. For the first time. a detailed infrared analysis of the spectral range between 1600 cm-1 and 1400 cm-1, which is of particular importance for carboxylate monolayers at the air-water interface, is presented. For all cations various bands representing the antisymmetric and symmetric carboxylate vibrations are observed indicating the heterogenic character of the monolayer. Ca2+ interacts mainly ionically with the carboxylate, although a small amount is presumably bound covalently. Cd2+ and Pb2+, however, are largely covalently coordinated with the carboxyl group at pH 6, resulting presumably in a chelating bidentate structure. It is proposed that upon reducing the pH value the Pb2+ chelate is broken. i.e. the carboxylate anion is being formed, and that finally the undissociated fatty acid becomes dominant.