Mixed Langmuir-Blodgett (LB) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C(18)TCNQ) and deuterated stearic acid (stearic acid-d(35)) were fabricated to investigate effects of incorporation of the fatty acid on molecular orientation and structure in LB films of C(18)TCNQ. A pi-A isotherm of the mixed Langmuir film suggests that C(18)TCNQ and stearic acid-d(35) are segregated in the film and that above 22 mNm(-1) the C(18)TCNQ domains start to collapse or the C(18)TCNQ molecules are squeezed out from the fatty acid monolayer on the water subphase. Ultraviolet-visible and infrared transmission and reflection-absorption (RA) spectra were measured for one-, three-, and five-layer mixed LB films deposited at 18 and 35 mNm(-1). A doublet peak caused by the CH2 scissoring mode of the alkyl chain of C(18)TCNQ suggests that the phase separation occurs in the mixed LB films. Infrared spectra of LB films of C(18)TCNQ alone give two bands at 2926 and 2918 cm(-1) due to the CH2 antisymmetric stretching modes of the partially disordered and highly ordered parts of the alkyl chain of C(18)TCNQ, respectively. In the corresponding spectra of the mixed LB films, the band at 2926 cm(-1) becomes very weak. Thus, the percentage of the partially disordered part is reduced markedly by mixing C(18)TCNQ with fatty acid. Molecular orientation of C(18)TCNQ is also changed significantly upon the formation of the mixed LB films. For example, the alkyl chain becomes more perpendicular, whereas the TCNQ plane becomes more parallel to the substrate surface. The reduction in the ratio of the partially disordered part of the alkyl chain and the change in the molecular orientation of C(18)TCNQ in the mixed LB films are probably due to freezing-in effects of the fatty acid domains which block the realignment of C(18)TCNQ during the film deposition. The infrared spectra of the mixed LB films deposited at 35 mNm(-1) show that the molecular orientation and structure of C(18)TCNQ change greatly because of the collapse of the C(18)TCNQ layer at this pressure.