Spectroscopic ellipsometry (SE) is used to study a sinusoidal-relief grating fabricated on a surface of transparent polymer. An optically thick polymer layer is situated on a glass substrate and its refraction index is optically matched to the index of the glass. The rigorous coupled-wave analysis, implemented as the airy-like internal reflection series, is applied to calculate the optical response of the relief grating. The entire optical response of the sample is determined by employing incoherent backreflections at the interface between the polymer and the glass. The parameters describing the dimensions and the real shape of the sine-like relief, as well as the quality of the optical matching between the polymer and the glass, are determined using SE together with atomic force microscopy as a complementary technique.