CaF2 layers were grown by molecular beam epitaxy on differently prepared Si(111) substrates. X-ray reflectivity spectra were measured and fitted. From the fitting process, the thickness of the CaF2 layer was precisely (within 1 Å) determined and the CaF2/Si interface roughness was also obtained. This roughness was used as an evaluation parameter for the quality of the layers. The CaF2/Si sample from which the intentional oxide was desorpted at 800°C inside the growth chamber exhibited the most clear x-ray reflectivity spectrum with very well resolved interference fringes. The epitaxial relations of the CaF2/Si samples grown at temperatures between 250 and 700°C were determined from x-ray diffraction analysis.