Magnesium fluoride layers are obtained by disproportionation of fluorine-containing organic magnesium compounds, notably magnesium trifluoroacetate, magnesium trifluoroacetylacetonate and magnesium hexafluoroacetylacetonate. A solution of such a compound is provided on the substrate in an organic solvent by spinning or dipping at room temperature after which the layer is heated to 500.degree. C. The solution may also be atomized and sprayed on the substrate which is maintained at a temperature of 600.degree. C. It is possible by means of this method to provide pure magnesium fluoride layers at a comparatively low temperature in air and without using film forming vehicles. The MgF.sub.2 layers obtained according to said method are useful as anti-reflective layers on display screens, on optical elements and as components in a multilayer packet, for example a dichroic mirror.