The feldspars Sr 0.05 Ba 0.95 Al 2 Si 2 O 8 , BaAl 2 Ge 2 O 8 and BaGa 2 Si 2 O 8 with S.G. I2/c, and BaGa 2 Ge 2 O 8 with S.G. P2 1 /a, were studied by means of crystal structural and microstructural analyses and dielectric measurements. All the investigated densely sintered single-phase feldspars exhibited a permittivity (ε) of 7-8 and a temperature coefficient of resonant frequency (τ f ) from -20 to -30 ppm/ • C. In contrast to the ε and τ f the dielectric losses were found to be dependent on the annealing conditions. In Sr 0.05 Ba 0.95 Al 2 Si 2 O 8 the Qxf values increased from 42,500 to 92,600 GHz when the annealing time at 1400 • C was increased from 1 to 162 h. Such a difference in the Qxf values as a result of various annealing conditions was attributed to different degrees of tetrahedral ordering. In contrast to aluminosilicate feldspars, Ge-containing feldspars can be sintered and ordered at low temperature. In BaAl 2 Ge 2 O 8 the Qxf values decreased when the sintering temperature exceeded the order-disorder I2/c ↔ C2/m phase-transition temperature. The BaGa 2 Si 2 O 8 and BaGa 2 Ge 2 O 8 feldspars exhibited a rapid decrease of Qxf values when the annealing temperature approached the melting point. However, the BaAl 2 Ge 2 O 8 and BaGa 2 Ge 2 O 8 can regain their high Qxf values by annealing at 1000 • C. The BaGa 2 Ge 2 O 8 stood out from the other investigated feldspars, with a sintering temperature of 1100 • C, Qxf values of 100,000-150,000 GHz and a τ f of -26 ppm/ • C.