The dielectric properties of dense ceramics of the "twinned" 8H-hexagonal perovskite Ba8Nb4Ti3O24 are reported. Single-phase powders were obtained from the mixed-oxide route at 1325 degrees C and ceramics (> 92% of the theoretical X-ray density) by sintering in air or flowing 02 at 1400 degrees-1450 degrees C. The ceramics are dc insulators with a band gap > 3.4 eV that resonate at microwave frequencies with relative permittivity, epsilon(r) similar to 44-48, quality factor, Q x f(r) similar to 21000-23500 GHz (at f(r) similar to 5.5 GHz) and temperature coefficient of resonant frequency, TCf similar to + 115 ppm/K.