A series of single-phase RBa1.5Sr0.5Cu3Oy (R = La, Nd, Sm, Eu, Gd, Dy, Ho, and Y) compounds with a triple-perovskite unit cell has been prepared by solid-state reaction method. Samples with a larger radius of the R ion (R = La and Nd) are tetragonal, whereas those with a smaller radius of the R ion (R = Sm, Eu, Gd,Dy, Ho, and Y) are orthorhombic. Unit-cell parameters of RBa1.5Sr0.5Cu3Oy decrease monotonically with a decrease of the radius of the R ion. A maximum at R = Nd, Sm, and Eu is found in the peritectic transition temperature (t(p)), the amount of oxygen loss (Delta w) in this transition, and the enthalpy of fusion (Delta H-f), respectively. An increase in the Delta H-f is correlated with increasing t(p) for the respective orthorhombic and tetragonal phases of RBa1.5Sr0.5Cu3Oy compounds. T-c of the title compounds gradually increased from 52 to 86 K with a decrease in the radius of the R ion.