Unique monolayer properties of a long-chain fatty acid, hexacosanoic acid (C26), have been studied by surface pressure-area (pi-A) isotherms, Brewster angle microscopy (BAM), and a new analytical technique using infrared extemal-reflection (IR-ER) spectroscopy, in comparison to other shorter long-chain fatty acids (C23-C25). The shorter fatty acids gave an identical limiting molecular area, although the shape of the isotherms was different from one another. Nevertheless, C26 yielded a significantly larger limiting molecular area than the shorter ones, with a relatively simple isotherm in shape. The IR-ER spectra of the C26 monolayer deposited on GaAs suggested that the molecules were ordered well in the film, which is not consistent with the large limiting molecular area. The dielectric dispersion analysis of the spectra, which has recently been developed, supported that the C26 monolayer had a highly ordered molecular conformation. The key to understanding the inconsistency was found in the degree of lateral molecular aggregation. The extraordinarily strong aggregation property has been suggested via the real part of the complex refractive-index analysis using the IR-ER spectra and confirmed by comparison of the C26 monolayer to the cadmium salt of the C26 monolayer using BAM.