The suppression effect of superconductivity by Pr substitution correlates strongly with that by oxygen deficiency in the R1-xPrxBa2Cu3O7-y system, where R is Yb, Y, Ho, Dy, Gd and Nd. The larger the value of x(C), the Pr content at which superconductivity disappears, the larger the value of y(C), the oxygen deficiency at which superconductivity disappears. The decreasing rate in T-C by Pr substitution ( -dT(C)/dx) decreases with decreasing ionic radius of a rare-earth ion for the Ortho-I phase (T-C > 90 K for x = 0) and also with decreasing oxygen concentration (7 - y) The - dT(C)/dx for the smaller rare-earth ions (R = Yb, Y, Ho and Dy) are nearly independent of ionic radius for the Ortho-II phase (T-C = 60 K for x = 0). The Pr content dependence of T-C in (Y0.625La0.375)(1-x)PrxBa2Cu3O7- y, average rare-earth ionic radius of which is equal to that of Gd, coincides well with that in the Gd1-xPrxBa2Cu3O7 -y. The weakened T-C-suppression by Pr in the case of the smaller rare-earth ions is explained by considering the hole transfer effect from the Cu(1)O chain toward the Cu(2)O-2 plane according to the difference in ionic radius between the rare-earth ions and Pr ion. (C) 2000 Elsevier Science B.V. All rights reserved.