Process for coating transparent substrates, for example float glass, with a transparent dielectric layer having a low refractive index (n 6.0A cm.sup.2 /W sec) by means of reactive direct current cathode sputtering. A vacuum chamber includes a cathode 5 which is provided on one of its surfaces with the material (target) to be sputtered and deposited on the substrate 3. The material to be sputtered (target) is a silicide, preferably nickel disilicide (NiSi.sub.2), and the layer deposited on the substrate is the corresponding oxide, for example NiSi-oxide. The reactive gas introduced into the vacuum chamber is oxygen and the process gas is a noble gas.