Positive secondary ion signals of the type Si n O + m ( n = 1–3, m = 1, 2) from a polycrystalline Si target under O + 2 bombardment in high vacuum at primary energies in the range 3–10 keV have been monitored as a function of target current density J p ( I p/ A, where I p is the target current and A the spot area). Most of the secondary ion clusters (but not the monomers) show pronounced chemical enhancement effects with an increase in I p. Moreover, the intensity ratios of the higher oxygen content clusters, e.g. SiO +/Si + or SiO +/Si 2O +, also showed a marked increase under the same conditions. Our results suggest that reactive primary atoms are gradually accumulated at the target surface under energetic ion bombardment at increased target current densities. The phenomenon stands rather in contradiction to the theoretical concepts where the surface stoichiometry is not expected to change with target current density once steady state is attained. Our observation also gives an insight into the cluster emission process at increased target current densities. It appears that our current ideas regarding the emission of small clusters need to be modified, at least in the high target current density regime.