Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and pi -A isotherms were applied to investigate in situ the fragile stearic acid/Ba2+ (BaSt) system on the aqueous BaCl2 subphase in a Langmuir trough. The spectra were recorded at 294-274 K, pH = 9.5 and 6.9, and 4 cm(-1) resolution with excellent signal-to-noise ratios. At pH = 6.3 the stearic acid was not totally deprotonated and no genuine unit cells were detected. The mixed COOH/COO- monolayer had CH2 antisymmetric (nu (as)(CH2)) and symmetric (nu (s)(CH2)) bands at 2917 and 2851 cm(-1), respectively, indicating a predominantly trans zigzag chain conformation. At pH = 9.5 the stearic acid was totally deprotonated after spreading and formed a monolayer with ionic interaction between the COO-group and the subphase Ba2+ ion. Later the monolayer spontaneously reorganized into a three-dimensional structure, where the COO group and the barium atom formed a chelating bidentate coordination. This reorganization took 11 min to complete at 294 K and 50 min at 274 K. The "reorganized films", either initially formed or tightly compressed, possessed genuine triclinic unit cells with a sharp singlet CH2 scissoring band at 1471 cm(-1) and all-trans zigzag chains with an unusually low nu (as)(CH2) frequency of 2914 cm(-1). These features disappeared if the "reorganized films" were left to relax at surface pressures below 1 mN/m.