Dielectric properties and phase formation of Bi-based pyrochlore ceramics were evaluated for the Bi2O3-ZnO-Ta2O5 system. The compositional range rBi(2)(Zn1/3Ta2/3)(2)O-7. (1-r)(Bi3/2Zn1/2)(Zn1/2Ta3/2)O-7 (0 less than or equal to r less than or equal to 1) in Bi2O3-ZnOTa2O5 was investigated to determine the relative solubility of BZT cubic (alpha -BZT, r = 0) and the pseudo-orthorhombic (beta -BZT, r = 1) end members. It was found that extrinsic factors, such as kinetically limited phase formation and bismuth loss, contribute to apparent phase boundaries in addition to thermodynamic stability of each phase. Considering this, the locations of true phase boundaries were r < 0.30 and r greater than or equal to 0.74 for alpha and beta phases, respectively. Dielectric constants between 58 and 80 and low dielectric loss (tan delta < 0.003) were measured for the complete compositional range. The temperature coefficient of capacitance was controlled by composition, which was found to be < 30 ppm/degreesC at the edge of beta -phase solid solution. In addition to the excellent dielectric properties these materials can be sintered at low temperatures, which make Bi-based pyrochlores promising candidates for high-frequency electronic applications.