Magnetization measurements have been made on polycrystalline samples of Y1-xPrxBa2Cu3Oy(y approximately 7,0.4 less-than-or-equal-to x less-than-or-equal-to 0.6) at 5 K. Two threshold Pr concentrations are identified: x(cl) approximately 0.46 and X(c2) approximately 0.56. As x increases towards x(cl), the initial susceptibility \dM/ dH\H=0 decreases sharply and irreversible entry of magnetic flux into thegrainsbegins to take place at a much lower field. The critical current density J(c), derived using the Bean model, decreases monotonically with x but the slope of the J(c) versus x curve is much larger for xx(cl). The diamagnetic signals almost disappearar at x(c2). These observations are discussed in terms of a 2D percolation model in which it is assumed that two Pr ions, when they occupy neighbouring rare earth sites, localize a hole on the oxygen site between them, locally destroying the superconductivity.