In the present work, we investigate in detail the effect of both the deposition temperature and the cooling rate on the growth of calcium barium niobate (CBN) thin films on a magnesium oxide (MgO) single crystal substrate. As revealed by x-ray diffraction measurements, epitaxy occurs for films deposited at 800 °C with a slow in situ cooling rate. We have also investigated the chemical composition of the films by way of a Rutherford backscattering spectroscopy technique and of an elastic recoil detection analysis, which reveal a good stoichiometry for our samples. In addition, optical analysis (UV-visible spectroscopy and spectroscopic ellipsometry) is performed on both polycrystalline and epitaxial films. A high transmittance (≫85%) and a refractive index close to that observed for bulk crystal are obtained for epitaxial CBN films deposited at a low oxygen pressure. X-ray reflectivity measurements have enabled us to correlate the density of the films with their refractive indices.