Deep level transient spectroscopy (DLTS) investigations have been used to characterize the electrical properties of interstitial clusters in ion-implanted Si. Both n- and p-type samples were implanted with 145 keV–1.2 MeV Si ions to doses of 1×1010–5×1013 cm-2 and annealed at 450–750 °C. On samples annealed at temperatures above 550 °C, the residual damage is dominated by two hole traps (B lines) in p-type and five electron traps (K lines) in n-type samples. Analyses of the spectra and defect depth profiles reveal that these signatures are related to Si self-interstitial clusters, and experiments confirm that these clusters do not embody large numbers of impurities such as C, O, B, or P. Four deep level signatures exhibit similar annealing behavior, suggesting that they arise from the same defect structure. On the other hand, the remaining signatures exhibit different annealing behaviors and are tentatively associated with different cluster configurations. We have found that the thermal stability of the clusters is enhanced by either increasing the Si dose or by reducing the impurity content of the substrate. The explanation of these effects proposes that bigger and more stable clusters are formed when the concentration of free interstitials available for clustering is increased and the competing interstitial trapping at impurities is inhibited. Finally, in samples implanted at doses of ≥1×1013 cm-2, most of the DLTS signals exhibit a complex and nonmonotonic annealing behavior providing evidence that the clusters can transform between electronic configurations. © 1998 American Institute of Physics.