Fourier-transform infrared reflection absorption (IRA) spectroscopy was applied to study the structures and UV-light-induced photochemical processes of a cinnamic acid derivative (C(22)CA) in Langmuir-Blodgett (LB) films on a silver substrate. The IRA spectra of the LB films of C(22)CA with the number of monolayers (L) of 1-9 prepared by changing surface pressures (10-30 mN/m) gave four sets of nu(C=O) and nu(C=C) bands at 1730 and 1641, 1684 and 1624, 1674 and 1624, and 1715 and 1638 cm(-1). The first set was assigned to the C(22)CA molecules forming a hydrogen-bonded chain of carboxyl groups or a "lateral hydrogen-bonded state", the second and third sets to "cis hydrogen-bonded dimers" (the cis conformation is defined in terms of the position of C=O and C=C groups around the C-C bond connecting the two groups), and the fourth set to the molecules forming a hydrogen-bonding between neighboring LB monolayers or a "face-to-face hydrogen-bonded state". The lateral hydrogen-bonded state exists only in a one-monolayer LB film (L = 1) and in the first monolayer of multimonolayer LB films (L greater than or equal to 3), while the cis hydrogen-bonded dimers are formed in the multimonolayer LB films assuming a structure similar to that of a crystalline state of C(22)CA. When the multimonolayer LB films were annealed at 100 degrees C, the face-to-face hydrogen-bonded state was converted to the hydrogen-bonded dimer with the trans conformation, giving the set of the nu(C=O) and nu(C=C) bands at 1694 and 1632 cm(-1), respectively. Upon UV irradiation, the C(22)CA molecules in the lateral hydrogen-bonded state do not show any photochemical process, while the molecules forming the cis and trans hydrogen-bonded dimers undergo photodimerization. On the other hand, the irradiation causes the conversion from the face-to-face hydrogen-bonded state to an irregular state consisting of the lateral hydrogen-bonded state and a non-hydrogen-bonded state. The conversion to the trans hydrogen-bonded dimer caused by the thermal treatment and the conversion to the irregular state due to the irradiation indicate that the face-to-face hydrogen bonded state is thermodynamically in a metastable (or a kinetically trapped) state. Thus, the IRA spectroscopy characterized a variety of structures of the cinnamic acid derivative forming LB monolayer assemblies and elucidated the correlation between the structures and photochemical processes induced by UV irradiation.