We have studied the microstructural changes occurring in YBa2Cu3O7-x (the so-called ‘123’ phase) upon room temperature irradiation by He+ and O +, in situ, on samples that had been thinned by ion milling before irradiation, using a transmission electron microscope on line with the ion implanter. In previous work on samples crushed from the bulk or on films ion-milled after irradiation, the samples amorphized above a threshold of about 0.3 displacement per atom. In the present case, no amorphization is observed, instead, precipitation of Cu2O occurs, indicating complete demixing of the ceramic. This process seems to nucleate at the ion-milled surfaces. For a displacement-per-atom ratio of about 0·10, a change from the orthorhombic to the tetragonal structure of the ‘123’ compound occurs. It is suggested that the result is quite general. The effects of the microstructural changes on the electrical properties are briefly discussed. © 1989 Taylor & Francis Group, LLC.