Results of field-ion microscopy investigations of formation and clustering of vacancies among themselves and intrinsic interstitial atoms with impurity atoms in tungsten and its slightly diluted alloys at irradiation by In+ or Yb+ ions of 25 keV energy and Ar+, Ni+, or Cr+ ions of 35 keV energy are presented. Distributions of vacancies complexes in irradiated samples over the number of individual vacancies associated in them have been obtained and analyzed. The noticeable difference of such distributions in areas of development of atomic displacement cascades (in depleted zones) and outside these areas has been revealed. Specific features of the distribution of identified complexes "intrinsic interstitial atom-impurity atom" (in the dumbbell configuration) over the volume of irradiated samples have been studied. The correlation between the type and concentration of components in alloys and concentration of these complexes (dumbbells) as well as dimensions and parameters of depleted zones has been established. The theoretical analysis of experimentally obtained regularities from the positions of the concept of the rupture of chains of focused replacing collisions on impurities has been carried out. Average values of chain lengths in samples with different levels and types of impurities have been measured indirectly. Effectiveness of capture of intrinsic interstitial atoms by various impurities in tungsten has been evaluated from these results. (C) 2003 American Vacuum Society.