The energy position of the optical-absorption edge and the free-carrier populations in Inx Ga1-x N ternary alloys can be controlled using high-energy He+4 irradiation. The blueshift of the absorption edge after irradiation in In-rich material (x>0.34) is attributed to the band-filling effect (Burstein-Moss shift) due to the native donors introduced by the irradiation. In Ga-rich material, optical-absorption measurements show that the irradiation-introduced native defects are inside the band gap, where they are incorporated as acceptors. The observed irradiation-produced changes in the optical-absorption edge and the carrier populations in Inx Ga1-x N are in excellent agreement with the predictions of the amphoteric defect model. © 2005 American Institute of Physics.