Abstract Oxides belonging to the families Ba 3ZnTa 2− x Nb x O 9 and Ba 3MgTa 2− x Nb x O 9 were synthesized by the solid state reaction route. Sintering temperatures of 1300°C led to oxides with disordered (cubic) perovskite structure. However, on sintering at 1425°C hexagonally ordered structures were obtained for Ba 3MgTa 2− x Nb x O 9 over the entire range (0≤ x≤1) of composition, while for Ba 3ZnTa 2− x Nb x O 9 the ordered structure exists in a limited range (0≤ x≤0.5). The dielectric constant is close to 30 for the Ba 3ZnTa 2− x Nb x O 9 family of oxides while the Mg analogues have lower dielectric constant of ∼18 in the range 50 Hz to 500 kHz. At microwave frequencies (5–7 GHz) dielectric constant increases with increase in niobium concentration (22–26) for Ba 3ZnTa 2− x Nb x O 9; for Ba 3MgTa 2− x Nb x O 9 it varies between 12 and 14. The “Zn” compounds have much higher quality factors and lower temperature coefficient of resonant frequency compared to the “Mg” analogues.