New, commensurate members of the fluorite-related Bi3Nb1-xTaxO7 family were synthesized and their crystal structures, microstructures, and microwave (MW) dielectric properties were characterized. The incorporation of Ta into the tetragonal Bi3Nb1-xTaxO7 solid solution was found to gradually affect the density and the MW dielectric properties. The materials sintered at 870 degrees C exhibited relative permittivities in the range k' = 86-72, Q x f values from 793 to 1189 GHz and a positive temperature coefficient of resonant frequency from 88 to 12 ppm/K. The formation of the members of the fluorite-related solid solution along the Bi3Nb1-xTaxO7 composition depends on a phase transition, and thus their properties are compared within the compositional range. The correlations between their MW dielectric properties, compositions, crystal structures, and processing parameters were discussed in detail. Optimization of MW properties can be achieved by utilizing the ability of the Bi3Nb1-xTaxO7 solid solution that it undergoes a phase transformation from cubic to tetragonal structure which are both characterized by unique properties, under certain synthesis conditions.