The effect of chemical substitution on the high-Tc (91 K) superconducting oxide YBa2Cu3O7, with the distorted perovskite structure, has been studied. Subtle changes in Tc occur when Y is partly replaced by Dy, Ho, Er, Tm, Yb, Lu, Bi and In. The possibility of coexistence of magnetic order and superconductivity or reentrant behavior in these high-Tc compounds will be described and discussed. © 1987.