[en] The increasing use of thin solid films for applications in the semiconductor technology of organic and inorganic materials, in optoelectronic devices (LED's, displays, switches), in magnetooptical storage etc., requires the characterization of their optical and electronic properties, e.g. with the aid of their optical constants, the refractive index and the absorption coefficient. The determination of optical constants from spectroscopic reflectance and transmittance measurements yields multiple solutions. It is shown that it is not possible to overcome the ambiguity from three normal-incidence measurements alone due to measurements errors, although the equation system is overestimated. In contrast to methods which require previous knowledge of the material, a method is proposed to determine the optical constants unambiguously from additional oblique-incidence measurements of a certain polarisation and from the oscillating behaviour of the spectra. The influence of measurements errors on the problem of ambiguity is discussed. (author)