Polycrystalline diamond thin films grown by hot-filament chemical vapour deposition (CVD) process were irradiated with 100 MeV I7+ ions. The as-deposited and irradiated films were characterized using micro-Raman spectroscopy, glancing angle X-ray diffraction (XRD), resistivity measurements and scanning electron microscopy (SEM) techniques. Defects and non-diamond carbon phases were seen to develop on irradiation up to a fluence of 1.3 X 10(14) ions/cm(2). The extent of damage to the films was found to be critically dependent on the crystalline quality of the film. The resulting changes were correlated with the hydrogen concentrations in the films as studied by elastic recoil detection analysis.