By utilizing the reflectance properties of water in the mid-infrared region of the electromagnetic spectrum, external reflection Fourier transform infrared spectroscopy is used to measure, in situ, the infrared spectrum of a monomolecular film at the air-water phase boundary. A monolayer of oleic acid (9-cis-octadecenoic acid) was spread on water to its equilibrium spreading pressure of 30 dyn/cm. The absorbance spectrum that resulted from the ratioing of the single-beam reflectance spectrum of oleic acid on water to that of pure water was found to have negative absorbance bands due to the properties of the complex refractive index of the water substrate. Vibrations are identified for both the hydrocarbon and carboxylic acid segments of the long-chain fatty acid monolayer. It is shown that the carboxylic acid remains protonated when the oleic acid exists as a monolayer film on water. © 1985 American Chemical Society.