12-Hydroxystearic acid is known to exhibit a wide plateau region between 0.3 and 0.95 nm(2/)molecule in the pi-A isotherm. In situ polarized FT-IR external-reflection spectra of Langmuir (L) films of 12-hydroxystearic acid on the water surface were recorded under various surface areas, and the molecular-orientation angles were quantitatively evaluated. At the final stage of the plateau region (0.4 nm(2)/molecule), the orientation angle of the hydrocarbon chain was about 55 degrees from the surface normal. Upon monolayer compression, however, the angle decreased to about 28 degrees in the solid-phase region. This change corresponds well to the result in Langmuir-Blodgett films that was reported earlier. In addition, it was elucidated that the hydrocarbon chain tended to become random during the surface pressure relaxation.