Fourier transform infrared attenuated total reflection (ATR) and UV-vis absorption spectra have been measured for one-monolayer Langmuir-Blodgett (LB) films of azobenzene-containing long-chain fatty acids (Figure 1, abbreviated as mAnH, m = 8 or 12 and n = 3 or 5) and their calcium (mAnCa), barium (mAnBa), or poly(allylamine) (mAnPAA) salts to investigate molecular aggregation, orientation, and structure and photoisomerization in the films. All the fatty acids and their salts investigated form H aggregates in the LB films. It is notable that the H aggregates of 8A3H and 8A5H decompose gradually with time, but such decomposition does not occur for 8A5Ca, 8A5Ba, 12A3H, and 12A3PAA. 12A3H forms much stronger H aggregates than 8A3H and 8A5H in the LB films. Infrared frequencies of CH2 antisymmetric and symmetric stretching bands of the alkyl chains of 12A3H and 8A3H in the LB films reveal that the films of both 12A3H and 8A3H have highly ordered hydrocarbon chains. Therefore, it seems that the strength of the H aggregates in the LB film of 12A3H is not concerned with the conformation of the alkyl chain but related to the existence of the oxygen atom bonded to the phenyl group. 8A3H and 8A5H photoisomerize from trans to cis or cis to trans configuration in the LB films upon the irradiation of LTV or visible light, respectively, even if fairly strong H aggregates are formed in the films. In contrast, 12A3H does not photoisomerize in the LB film, and even 12A3PAA, where the formation of polyion complexes with PAA widens largely the molecular area, photoisomerizes a little. These observations suggest that the length of the alkyl chain is an important factor for the feasibility of the photoisomerization. Of particular note in the structure of LB films of 8A5H, 8A3H, and 12A3H is that the carboxylic group of 8A5H and 12A3H molecules forms hydrogen bonds with those of their neighbors.