[en] In the entire series of rare earth compounds isomorphic to YBa₂Cu₃O_7-y, only PrBa₂Cu₃O_7-y is not a superconductor. In fact, this material exhibits insulating behavior at all temperatures. Various explanations have been given for this unusual property, the most common being the speculation that in the 1-2-3 structure, Pr may indeed be tetravalent, rather than trivalent as is Y and the other lanthanide substitutions. If this were true, then no excess positive charge would be present in stoichiometric PrBa₂Cu₃O_7-y to produce metallic conduction and hence superconductivity. In order to probe this speculation, we undertook to study the effect of oxygen concentration on the structural and physical properties of PrBa₂Cu₃O_7-y, paying particular attention to possible changes in Pr valency. Throughout most of this paper, unless otherwise indicated, the terms valency and ionicity will be used interchangeably