The microstructure of ion-implanted thin films of the superconductor YBa2Cu3Ox has been investigated by transmission electron microscopy. The superconducting properties of the films were dominated by large pancake-shaped grains of YBa2Cu3Ox with their c axis perpendicular to the substrate. Other grains of YBa 2Cu3Ox whose c axis was parallel to the substrate formed spherulites. Irradiation with 500 keV O+ ions caused amorphous zones to appear on the grain boundaries between the pancake grains, which initially were free of amorphous or second phases. At higher dose a continuous amorphous layer 150 Å thick was formed. However, the interior of the grains showed no irradiation-induced microstructural features until they became amorphous at a dose of 3×1014 ions/cm 2. The appearance of the amorphous layer on the grain boundaries at low doses accounts for the reduction in the superconducting transition temperature observed in these films.