The effects of M (M = Zn and Ni) substitution for Cu in R2BaCuO5 (R = Y and Yb) on the microwave dielectric properties and the fort-nation of the solid solutions were investigated in this study. The Y2Ba(Cu1-xZnx)O-5 and Yb2Ba(Cu1-xNix)O-5 solid solutions were single phase, whereas the limits of Y2Ba(Cu1-xNix)O-5 and Yb2Ba(Cu1-xZnx)O-5 solid solutions were x = 0.75. The dielectric constant (epsilon(r)) of Y2Ba(Cu1-xZnx)O-5 and Yb2Ba(Cu1-xNix)O-5 solid solutions vary from 7 to 16. The quality factor (Q) is extremely improved by M substitution for Cu, and the maximum value is 11215 at 10.162 GHz. (C) 2002 Elsevier Science B.V. All rights reserved.