Phase studies of solid solutions based on (Bi1.5Zn0.5)(Ti1.5Nb0.5)O-7 revealed extended regions of pyrochlore formation in the Bi2O3-ZnO-TiO2-Nb2O5 system. At room temperature and 1 MHz (Bi1.5Zn0.5)(Ti1.5Nb0.5)O-7 has a high permittivity (kappa＇ = 200), low dielectric loss (tan delta < 1 . 10(-4)) and a temperature coefficient of the permittivity, tau(kappa＇), = -1300 ppm/K. Pyrochlore solid solutions based on (Bi1.5Zn0.5)(Ti1.5Nb0.5)O-7 can be formed with (Bi-2- xZn(x))(Ti2- x Nb-x)O-7 (0.35 less than or equal to x less than or equal to 1.0) and with (Bi-1.5 Zn0.5-y/3Ti1.5+yNb0.5-2y/3) O-7 (-1.5 less than or equal to y less than or equal to 0.75). Investigations of the dielectric characteristics showed that the high temperature dependence of the permittivity of (Bi-1.5Zn(0).5)(Ti-1.5Nb(0).5)O-7 can be significantly modified by changing the composition of the pyrochlore within these regions of solid solubility. Below room temperature several of these compositions also exhibit a diffuse frequency dependent maximum in their permittivity characteristic of a transition to a relaxor type ferroelectric state. A third region of high permittivity pyrochlores with (Bi1.5+2zZn0.25-zTi2.25-z)O-7 (0.0 less than or equal to z < 0.15) was also identified in the Bi2O3-ZnO-TiO2 sub system.