Dense, single-phase Sr x Ba 1−x Al 2 Si 2 O 8 (0≤x≤1) and Ca y Ba 1−y Al 2 Si 2 O 8 (0≤y≤0.05) ceramics were synthesized and then structurally and dielectrically characterized. The substitution of Ba with Ca or Sr promotes the transformation of hexacelsian to monocelsian. The substitution with Ca led to a more-than-two-times-lower Q×f value at y=0.05, whereas the Q×f value of the Sr 0.05 Ba 0.95 Al 2 Si 2 O 8 (x=0.05) was only 5% lower than the Q×f value of the BaAl 2 Si 2 O 8 , which exhibited Q×f values of 70,600 and 77,700GHz when sintered at 1500°C for 12 and 40h, respectively. The improvement in the Q×f value with the prolongation of the heat-treatment time, which can be attributed to an increase in the Al:Si ordering, was also noticed for other Sr x Ba 1−x Al 2 Si 2 O 8 solid solutions. In contrast to the dielectric losses, the permittivity (ɛ=7.3 and 7.2 at 1MHz and 10.5GHz, respectively) did not change significantly with the composition and the sintering conditions.