Recent studies have shown evidence for the aggregation of self-interstitials in ion-implanted Si, resulting in nanoscopic damage structures. Similarly, selfinterstitial atoms are expected to play an important role for defect clustering in ion-implanted GaAs. Accurate first-principles total-energy calculations are reported for different As and Ga self-interstitial configurations. These results are used to study by first-principles total-energy calculations the structural and electronic properties of small complexes involving self-interstitials and/or antisites.