Six sillenite compounds Bi(12)MO20-delta (M = Si, Ge, Ti, Pb, Mn, B1/2P1/2) were synthesized, and the resulting single-phase powders were then sintered to obtain similar to 97% dense ceramics. An analysis of their microwave dielectric properties, performed at similar to5.5 GHz, revealed a permittivity of similar to 40 for all six compounds. The temperature coefficient of resonant frequency was the lowest for the Ph analogue (-84 ppm/K) and was found to increase with increasing ionic radius of the B-site ion to a value of -20 ppm/K for the Bi12SiO20 and Bi-12(B1/2P1/2)O-20 compounds. The Q x f value is a maximum for Bi12SiO20 and Bi12GeO20 with 8100 and 7800 GHz, respectively. The dielectric properties of the sillenites have been correlated with the structure of the oxygen network of the sillenite crystal lattice. As a result of its low sintering temperature (850 degreesC), chemical compatibility with silver, low dielectric losses, and temperature-stable permittivity, the Bi12SiO20 compound is a suitable material for applications in low-temperature cofiring ceramic (LTCC) technology.