ZrO2-MgO optical coatings were deposited by reactive thermal coevaporation. The nickel substrate optical constants were determined by ellipsometry. Film reflectance measurements at normal incidence were made in the region 350 nm-800 nm in order to determine the Fresnel reflection coefficient. The film’s refractive index was computed using the Fresnel formalism. It was found that it varies with the amount of MgO in the alloy. The maximum values were 2.29 +/- 0.12, 2.18 +/- 0.10 and 2.06 +/- 0.07 respectively for lambda = 0.37 mu m, lambda = 0.63 mu m and for lambda = 0.75 mu m. These maxima were obtained from the sample containing 15 mol.% MgO in the alloy. At molar percentages of MgO greater than 20% a monotonic decrease of index is noted. The investigated samples displayed a large dispersion of film refractive index. We found a film packing density ranging from 0.75 to 0.98. As a function of MgO mole fraction, this important factor related to film structure determines the refractive index behaviour. These investigations aimed to produce thin optical coatings with variable optical constants for optimizing the performance of an optical device for a certain wavelength or wavelength region.