The use of sliding X ray beams for studying the substructure (the crystallite dispersion and the dislocation density) of surface layers having a considerably inhomogeneous structure in depth is presented. Comparative data on the physical broadenings (β) of the interference lines, which have been defined by applying sliding X ray beams and symmetrical measuring (in Bragg–Brentano geometry), of metal surfaces modified through electrospark alloying are presented. The significant difference in the β values is shown. This difference explains the possible inconsistency in the subsequent calculations of the substructural parameters in the case of measurements in another mode.