<p>Mg-substituted bismuth niobates codoped with Group 13 elements (Al³⁺, Ga³⁺, and In³⁺) were synthesized. The samples are characterized by a pyrochlore-type structure. X-ray diffraction analysis was used to refine the crystal structure of the compounds, the distribution of Mg²⁺ cations at the B sites and p-element cations at the A and B sites in equal parts was determened. The diffuse reflectance spectroscopy was used to determin the band gap for the direct electronic transition (<i>E</i>_g^dir = 3.2–3.3 eV). The impedance spectra were analyzed: the dielectric characteristics and the conductivity type were determined. The compounds display dielectric behavior up to a temperature of 200 °C, and oxygen transport occurred above 400 °C.</p>