A study of K x Ba 1−x Ga 2−x Ge 2+x O 8 ceramics revealed that these solid solutions undergo a monoclinic-to-monoclinic P2 1 /a⇔C2/m phase transition. The temperature of this phase transition decreases with an increase in x, in a similar way to the sintering temperature, which decreases from 1100°C (x=0) to 970°C (x=1). The temperature of the P2 1 /a⇔C2/m phase transition is below the sintering temperatures of K x Ba 1−x Ga 2−x Ge 2+x O 8 (0.67≤x≤1) solid solutions, whereas the compositions at lower x (x=0.4 and 0) remain in the P2 1 /a modification over a wide temperature range above the sintering temperature. Compared to the C2/m modification of K x Ba 1−x Ga 2−x Ge 2+x O 8 (0.67≤x≤1), with a permittivity of 6.2–6.9, the P2 1 /a modifications exhibit permittivities of 5.9–7.0 and three-to-eight times higher Q×f values of ∼100,000GHz (at ∼12GHz). The temperature coefficient of the resonant frequency is ∼−25ppm/K, regardless of the composition.