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Dielectric properties of both Nd/sub 1.9/Ba/sub 1.1/Cu/sub 3/O/sub 7+/spl delta// (NBCO-213) and Pr/sub 1.14/Ba/sub 1.86/Cu/sub 3/O/sub 7-/spl delta// (Pr-rich PBCO) single crystals have been examined at low temperature. These materials have good lattice matching to high-T/sub c/ superconductors (HTS), but they are conductive at room temperature. Below 80 K, they are insulators with low dielectric constants, /spl epsi//sub /spl tau// below 25, and low dielectric loss tan /spl delta/ below 0.1 at 100 kHz. The value of /spl epsi//sub /spl tau// is suitable for insulators in integrated circuits using strip line widths of 10 /spl mu/m order, providing short delay time, no excitation of surface wave, and low radiation loss. The value of tan /spl delta/ is comparable to loss of superconducting surface resistance above 100 GHz. These results indicate the applicability for the insulator layers in multilayer superconducting electronic devices such as Single Flux Quantum (SFQ) circuits operated at high speed.