Abstract Optical anisotropy of Langmuir–Blodgett films and its changes during polymerization under the action of UV irradiation was investigated by means of multiangle ellipsometry with fixed wavelength 632.8 nm. We investigated the films of Cd and Pb salts of acetylenic acids with the terminal and internal triple bond: HCC(CH 2) 21COOH (1) and CH 3(CH 2) 18CCCOOH (2). Using the model of biaxial anisotropic film we determined optical parameters (refractive indices and thickness) of the films under investigation. It was established that the films of Pb salt of acid (2) should be considered as biaxial, while the films of Cd salt of acid (2), as well as Cd, Pb salts of acid (1) are nearly uniaxial. The calculated monolayer thickness values coincide with the values determined by means of small-angle X-ray scattering. It is demonstrated that the anisotropy of the films changes under the action of UV radiation. After polymerization, the anisotropy in the plane of the layer almost disappears while it is conserved in the direction perpendicular to the interface. A method is proposed for determining the parameters of thin films, which allows determining the changes in optical properties of the films with a thickness of 5 nm and more.