Nanoscale clusters are often formed during heteroepitaxial crystal growth. Misfit between the lattice parameter of the substrate and the adsorbate stimulates the formation of regular clusters with a characteristic size. The well-known "hut-clusters" formed during the growth of Ge on Si(001) are a good example of this type. Adsorbates can also produce another type of nanocluster; if the surface free energy of a particular crystallographic plane becomes lower than that of the geometrical surface of the substrate, then the entire surface will break up into regular arrays of small facets which look similar to the "hut clusters". We demonstrate that X-ray diffraction in combination with scanning tunneling microscopy can be used to determine the fundamental properties of such clusters. (C) 1998 Elsevier Science B.V. All rights reserved.