We have performed systematic studies of the effects of high-energy particle irradiation on the properties of InGaN alloys. In agreement with the amphoteric defect model, irradiation of InN produces donor-like defects. The electron concentration increases with increasing radiation dose and saturates at 4 X 10(20) cm(-3) at very high doses. We find that the increase of the electron concentration causes a large blue-shift of the absorption edge, which is well explained by the Burstein-Moss effect. The maximum electron concentration decreases with increasing Ga fraction in irradiated In1-xGaxN alloys as the conduction band edge approaches the Fermi level stabilization energy (E-FS). For x > 0.66 the conduction band edge moves above E-FS and the irradiation of n-type films produces acceptor-like defects, resulting in a reduced free electron concentration. An analysis of the concentration dependence of the electron mobility in InN indicates that the dominant defects in irradiated InN are triply charged donors. Finally, we show that InN films doped with Mg acceptors behave like undoped films above a threshold radiation dose. (c) 2006 Elsevier B.V. All rights reserved.