A new class of complex perovskites REBa₂ZrO_5.5 (where RE = La, Ce, Eu, and Yb) have been synthesized and sintered as single phase materials by the solid state reaction method. The structure of these materials was studied by x-ray diffraction, and all of them were found to be isostructural, having a cubic perovskite structure. X-ray diffraction and resistivity measurements have shown that there is no detectable chemical reaction between YBa₂Cu₃O_7–delta; and REBa₂ZrO_5.5 even under severe heat treatment at 950 °C, and that the addition of REBa₂ZrO_5.5 up to 20 vol.% in YBa₂Cu₃O_7–δ shows no detrimental effect on the superconducting properties of YBa₂Cu_{3O7-δ. Dielectric constants and loss factors are in the range suitable for their use as substrates for microwave applications. Thick films of YBa2Cu3}O_7–δ fabricated on polycrystalline REBa₂Zr_O5.5 substrates gave a zero resistance transition temperature T_c(0) ∼ 92 K, indicating the suitability of these materials as substrates for YBa₂Cu₃O_7–δ.