An overcoat protected diffraction grating. A replica grating having a thin aluminum reflective grating surface is produced by replication of a master grating or a submaster grating. The thin aluminum reflective surface may be cracked or have relatively thick grain boundaries containing oxides and hydroxides of aluminum and typically is also naturally coated with an aluminum oxide film. The grating is subsequently overcoated in a vacuum chamber with one or two thin, pure, dense aluminum overcoat layers and then also in the vacuum the aluminum overcoat layer or layers are coated with one or more thin protective layers of a material transparent to ultraviolet radiation. In preferred embodiments this protective layer is a single layer of MgF2, S1O2 or Al2O3. In other preferred embodiments the layer is a layer of MgF2 or S1O2 covered with a layer of Al2O3 and in a third preferred embodiment the protective layer is made up of four alternating layers of MgF2 and Al2O3 or four alternating layers of S1O2 and Al2O3. Preferably, the thickness of the transparent protective layers are chosen to produce a phase shift at the proposed operating wavelengths of an integral number of 2π.