Ellipsometric parameters as functions of the angle of incidence of light on a film for three thin films of molybdenum at the excitation of surface polaritons are experimentally measured with the use of the Beattie method and are theoretically calculated within the three-layer model of thin film. The measured ellipsometric parameters are the azimuth of a restored linear polarization and the phase shift between the $p$- and $s$-components of reflected light. Theoretical calculations are performed by the Airy recurrent formulas. The studies carried out show that the experimental and theoretical dependences of the ellipsometric parameters on the angle of incidence have the identical behaviour, but some quantitative differences are revealed. It can be explained by assumption that the interfaces between layers are ideally planar in the theoretical three-layer model of thin Mo film. At the same time, the real boundaries are rough and inhomogeneous. The results obtained demonstrate that the middle layer of film is metallic molybdenum, and the upper and lower layers are molybdenum oxides.