Molecular adsorbates of stearic acid on a gold surface prepared as an imperfect Langmuir-Blodgett (LB) film is found to exhibit dynamic molecular rearrangement when the humid atmosphere about the sample is changed. The molecular adsorbates stored in a thoroughly dried sample room of FT-IR is found to have a unique adsorption structure; the hydrocarbon chains have a nearly parallel orientation to the substrate surface while the molecules are highly packed to have the orthorhombic subcell packing, which is confirmed by infrared reflection-absorption (RA) spectrometry. When the sample is pulled into an ambient air, the adsorption structure exhibits a drastic change in about only 15 min, which is pursued by polarization-modulation infrared reflection-absorption spectrometry (PM-IRRAS). The spectra clearly indicate that the molecular stance has largely been changed to have a standing-up orientation, whereas the molecular conformation is largely degraded. When the sample is got back to the dried sample room, the molecular conformation largely improves while the standing orientation is kept. These irreversible changes are induced by ambient water molecules adsorbed on the lying stearic acid molecules, which was monitored by analyzing absorption bands of the hydronium ion.