In a mask (7) for covering the outer marginal area of a disk-shaped substrate surface during a coating process, for example a vacuum sputtering or vapor depositing process, the mask (7) is made of an elastic material in the form of a planar plate with an essentially circular opening. To center and lock it onto the substrate (1) an inwardly extending annular flange is provided which assumes the actual masking function and extends in a plane parallel to the plane of the plate. The substrate (1) is laid on this flange and retained by projections extending radially inward. Prior to positioning the substrate it is bowed axially to spread apart the projections.