The ordered double perovskites ALaMgTaO 6 (A ΒΌ Ba, Sr, Ca) and La 2 Mg(Mg 1/3 Ta 2/3 )O 6 have been prepared and characterized. Synchrotron X-ray powder diffraction analyses show that all four compounds exhibit a rock-salt type ordering of the B-site cations (Mg 2+ /Ta 5+ ) and a random distribution of A-site cations (A 2+ /La 3+ ). The space group symmetries are determined to be Fm 3m for BaLaMgTaO 6 , and P2 1 /n for SrLaMgTaO 6 , CaLaMgTaO 6 , and La 2 Mg(Mg 1/3 Ta 2/3 )O 6 . Diffuse-reflectance spectroscopy shows these ordered perovskites have optical band gaps in the range of 4.6Γ4.8 eV. These values are roughly 1 eV wider than the ternary perovskite oxides of Ta 5+ such as KTaO 3 , due to narrowing of the conduction bandwidth which results from Mg 2+ /Ta 5+ ordering. These compounds are insulators with dielectric permittivities of k ΒΌ 18-23, dielectric losses of tan d ΒΌ 0.004-0.007, and small temperature coefficients of capacitance o100 ppm/K over the temperature range 20-150 1C. BaLaMgTaO 6 is of particular interest because it possesses a near-zero temperature dependence of capacitance.