Tilt angles of nematic liquid crystal molecules, 5CB, have been investigated in the thick cells having aligning layers of polyimide (PI) Langmuir-Blodgett (LB) films on aluminum electrodes using the attenuated total reflection (ATR) measurement method. The ATR curves strongly depended upon AC voltages applied to the cells and the properties at each voltage were similar between 17 and 25 degrees C. The resonant excitations of the surface plasmon polariton (SPP) were observed in a broad region of the incident angle in the ATR measurements. The tilt angles of the liquid crystal molecules were calculated by fitting the theoretical ATR curves to the experimental ones. The results showed that the ATR measurement method was very useful for evaluating behaviors of liquid crystal molecules in thick cells. (C) 1999 Elsevier Science S.A. All rights reserved.