The penetration of X-ray radiation into magnetic tape can be used to measure the pigment loading in the tape. An X-ray beam, for example, Fe K alpha radiation, is directed through an Mn filter and diffracted from a silicon single crystal (111) plane and deflected through a small beam aperture, for example, a 50-mil aperture, onto the tape surface where it is transmitted to a detector. The pigment loading can be determined from an analysis of the difference between the intensity of the beam after passing through the tape before loading with pigment and after loading with pigment.