Infrared reflection-absorption spectroscopic (IRRAS) measurements of octadecanoic acid monolayers on aqueous subphases containing 1 mM BaCl2, CuCl2, NiCl2, and ZnCl2, respectively, supplied insight into the molecular order and the coordination complexes of the system alkanoic acid/bivalent cation. The formation of highly ordered crystalline domains already at large areas per molecule is reflected by antisymmetric methylene stretching vibrations which are independent of the compression status, however, cation-dependent (Ba2+, Ni2+, similar to 2917 cm(-1); Cu2+, 2916.1 cm(-1); Zn2+, unusually low, 2914.3 cm(-1)). The analysis of the antisymmetric and symmetric carboxylate stretching vibrations allowed the determination of coordination types of the respective alkanoic acid/cation system, while from the methylene scissoring vibration the subcell structure could be inferred in most cases. Ba2+ cations form an ionic alkanoate, the chain packing of which appears to be mainly orthorhombic. For CU2+ cations both bridging bidentate and chelating bidentate complexes were found, and the chain packing is hexagonal. Ni2+ cations appear to show both monodentate and ionic complex formation, while for Zn2+ cations a single species was observed which is assigned to an unsymmetric chelating bidentate complex and a triclinic chain packing. Furthermore, the carboxylate and methylene scissoring bands allowed elucidation of the transformation mechanism that is encountered upon successive in situ acidification of the octadecanoic acid monolayer/Zn2+ system : in addition to the unsymmetric chelating bidentate complex (triclinic) encountered at pH 6.7, an ionic interaction becomes increasingly evident between pH 6.1 and 5.55, and eventually at pH 5.0 the free alkanoic acid (orthorhombic?) is the dominant species.