A detailed investigation on phase transitions in Langmuir monolayers of 5'-phenyl-1,1':3'1 "-terphenyl-4-carboxylic acid (PTCA) at the air-water interface is presented. Surface pressure and surface potential isotherms were obtained using varying experimental parameters such as spreading volume, subphase pH, ionic strength, subphase temperature, and compression speed. The isotherms exhibit two liquid-condensed regions separated by a broad plateau with a 2-fold decrease in area/molecule. PTCA monolayers were visualized by Brewster angle microscopy, transferred as multilayer LB films and characterized by Fourier transform infrared spectroscopy, X-ray diffraction (XRD), and surface potential measurements. On the basis of the analysis of surface pressure and surface potential isotherms, it was not possible to distinguish between bilayer formation and conformational changes of the triphenylbenzene ring system upon compression as the origin of the plateau. The LB transfer-from monolayers in the plateau was unsuccessful and thus characterization of LB films does. not help in elucidating the possible cause for the transition.