The discovery of a class of oxides showing superconducting properties at temperatures of 38 K. and above has generated intense interest. We have studied the effects of ion implantation into thin films consisting largely of one of these oxides, YBa2Cu3Ox, which typically has a superconducting transition temperature, Tc of greater than 90 K. Ion implantation with 500 keV oxygen ions causes degradation of the completion temperature of the superconducting transition, and changes the room temperature electrical properties from conducting to insulating. Further irradiation causes a sudden crystalline to amorphous structural transition. In the case of arsenic ion bombardment the c-a transition was more gradual. These effects were studied using TEM and X-ray diffraction to characterize the structure of the films. The deposited energy required to change Tc was found to be 0.2 eV/atom, and the change was ascribed to the formation of an amorphous layer on the grain boundaries. 1 to 2 eV/atom was required to affect the room temperature conductivity, and 4 eV/atom to render the film amorphous. The damage mechanisms responsible for these effects are discussed. © 1988.