Molecular orientation and structure in one-layer Langmuir-Blodgett (LB) films of octadecyldimethylamine oxide (C(18)DAO) and dioctadecyldimethylammonium chloride (2C(18)DAC) were investigated by use of infrared (IR) transmission, reflection-absorption (RA), and attenuated total reflectance (ATR) spectroscopy. The IR measurements suggest that the alkyl chains of the one-layer LB film of 2C(18)DAC on a gold-evaporated glass slide are highly ordered, whereas those of the C(18)DAO LB film have some gauche conformers. The molecular orientation and structure in both LB films show significant substrate dependence. Probably, the headgroup-substrate interaction controls them. Aging effects on the LB and Langmuir (L) films on an air-substrate interface were studied for the two kinds of amphiphiles. The alkyl chains of C(18)DAO in the LB film on the gold-evaporated glass slide become more ordered with time whereas the reverse trend is observed for those of 2C(18)DAC in the film. The effects on the L films were investigated by measuring IR spectra of the LB films prepared from the L films at various stages. The alkyl chains are less ordered in the LB films of C(18)DAO and 2C(18)DAC prepared from the aged I, films. It is also noted that the molecular orientation and structure in the films of both compounds show strong pH dependence, presumably because of the strong interaction between the headgroups and substrates.