High energy light ion irradiation has been used to anneal H-related defect complexes and to modify the electronic properties of semi-insulating InP (SI-InP). Raman and infrared spectroscopic measurements have been used to investigate the annihilation of native defects in SI-InP irradiated with 85 MeV C. Irradiation resulted in a decrease in sample resistivity by four orders of magnitudes and a change in the type of conductivity. The Raman spectroscopic results indicate an improvement in the InP sample due to irradiation up to an optimum fluence. The role of high electronic energy loss in defect annealing, which includes modification of the electrical properties and crystal structure of irradiated SI-InP, is discussed. (c) 2008 American Institute of Physics.