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Abstract This paper reports previously unpublished results which were obtained in 1966. We systematically investigated the dependence of cluster ion intensities on the bombarding gases He+, Ar+ and Xe+ (energies: 4 to 12 keV, current densities: 100 mA/cm2). Frequently, the observed structures in the relative cluster intensities were quite puzzling, e.g. for A1 and Si. Attempts to correlate these structures to crystal configurations failed, nor did any pattern develop from simple valency considerations alone. Initial ion energy distribution measurements from 0 to 1200 eV showed significant differences for atomic ions and cluster ions. This effect is used to reduce interference problems caused by cluster ion peaks in SIMS applied to trace analysis of solids. The results are discussed and compared with those of other investigators, also including cluster formation by vaporization and sparking. Extending known theoretical considerations may possibly afford a general understanding of the intensity structure. The formation and ejection mechanisms of clusters, however, remain unknown. Thermal effects to explain the latter are definitely discounted by the magnitude of the observed initial cluster energy (> 10 eV). Also discussed are two phenomena which demonstrate the presence of the bombarding gas in the surface.