Our analysis of the microwave dielectric properties of the delta-Bi2O3-Nb2O5 solid solution (delta-BNSS) showed a continuous increase in permittivity and dielectric losses with an increasing concentration of Nb2O5. The only discontinuity was found for the temperature coefficient of resonant frequency, which is negative throughout the entire homogeneity range but reaches a minimum value for the sample with 20 mol% Nb2O5. At the same composition there is a discontinuity in the grain size of the delta-BNSS ceramics. For the sample containing 25 mol% Nb2O5 two structural modifications were observed. A single-phase tetragonal Bi3NbO7, in the literature referred to as a Type-III phase, is formed in a very narrow temperature range from 850degrees to 880degreesC. A synthesis performed below or above this temperature range resulted in the formation of the end member of the delta-BNSS homogeneity range. Compared with the delta-BNSS the Bi3NbO7 ceramics exhibit lower microwave dielectric losses, an increased conductivity, and a positive temperature coefficient of resonant frequency.