A series of single phase La1.5-x/2SrxBa1.5-x/2Cu3Oy (0 less than or equal to x less than or equal to 0.80) compounds with a tetragonal triple-perovskite unit-cell have been prepared by solid-state reaction. With increasing x, unit-cell alpha axis, volume, density, and peritectic transition temperature (T-p) of La1.5-x/2SrxBa1.5-x/2CU3Oy decrease monotonically. The peak position of T-p measured in the 25 mL/min flowing O-2 atmosphere and 10 degrees C/min heating rate decreases from 1105 to 1080 degrees C as x increases from 0 to 0.80. The oxygen atom loss in the peritectic transition is 0.38 +/- 0.05 O-atom . mole(-1).