The spontaneous and nonlinear dielectric polarization of Langmuir monolayers is expressed using orientational order parameters. With the theoretical calculations, the Maxwell displacement current (MDC) measurement coupled with optical second harmonic generation (SHG) measurement is shown to be helpful for the study of orientational orders and phase transitions in monolayers. Spontaneous and nonlinear polarization induced in monolayers are detected by MDC and SHG, respectively. Using the MDC-SHG measurement, monolayers of 4'-n-octyl-4-cyanobiphenyl on the water surface were examined by monolayer compression. The phase transitions such as from planar isotropic phase to polar and tilting orientational alignment phases were clearly revealed. (C) 2003 American Institute of Physics.