A direct determination of the metrical thickness of mono- and multi-molecular films of barium stearate has been carried out with the use of multiple beam interferometry. The thickness of the unimolecular film, as obtained from measurements on a four-layer film, is 25.7 ± 0.3 Å which is in excellent agreement with the length of the molecular chain of barium stearate (=25.75 ± 0.25 Å) as obtained from Fankuchen's X-ray data. The technique employed eliminates differential phase change effects and does not involve the refractive index of the films. The result confirms the monomolecular spreading of stearic acid on a water surface.