The appearance of superconductivity in some PrBa(2)Cu(3)Q(7) (y similar to 7) single crystals and thin films can be accounted for on the assumption of the presence of disorder in the Ba/Pr site occupancy. It is argued that Pa ions occupying the Pr site and Pr ions occupying the Ba site both weaken Pr-O hybridisation through local structural changes and lower the Madelung potential at the Cu site relative to that at the Pr site, thus favouring the Cu-III oxidation state over the Pr-IV oxidation state. Bulk superconductivity shows up when local superconducting regions, created near the defects, percolate through the sample. It is concluded that stoichiometric, defect-free PrBa2Cu3O7 is insulating. (C) 1999 Published by Elsevier Science B.V. All rights reserved.