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The formation of metal clusters with hundreds of atoms in and near vapor ejection nozzles is analyzed. A previously discussed model, cluster generation by homogeneous nucleation and collisional growth in the vapor phase, is found to be untenable because it cannot account for the large number of collisions required to remove the latent heat of condensation, freed during cluster growth. An alternative mechanism that involves heterogeneous nucleation (i.e., cluster formation on surfaces) is proposed and evaluated. To determine the applicability of this mechanism, the following process steps are considered: formation of an adequate supply of critical embryos on nozzle and adjacent crucible walls, growth of embryos into large clusters, and cluster ejection into the surrounding vapor atmosphere. The conclusion is that, with nonwetting vapor and surface material combinations, substantial quantities of large clusters can be formed and ejected. Specifically, predictions for silver vapor at a few Torr pressure, passing through a graphite nozzle at about 1600 K, are cluster generation rates of 1013–1014 clusters/cm2 s, and cluster sizes up to about 600 atoms. Available experimental observations support these predictions qualitatively.