A polycrystal tungsten target (250°C) was bombarded with 150 keV ions of He, Ne, Ar, Kr, Xe, and Cu (current density 200 μA/cm2). With a mass and energy spectrometer the mass distribution of the sputtered charged particles was determined in the energy range 10 to 1250 eV. With increasing projectile mass the yield and the observable size of clusters Wn+increased. In the present study this yield decreased monotonously with increasingn. The maximum cluster size observed was W+12during sputtering with Xe+. The cluster yield decreased steeply with increasing cluster size and cluster energy. The slope of this energy dependence, however, appeared to be independent of the projectile mass. A crude kinetic model based on collision cascades is proposed to account for the cluster formation.