Samples of CU(0.10 - x)-Mg-x doubly substituted bismuth vanadate, BICUMGVOX (Bi2Cu0.10 - xMgxV0.9O5.35) were synthesized by the citrate sol-gel method. The influence of Cu-Mg substitution on phase stability and oxide-ion performance have been investigated using X-ray powder diffraction (XRPD), FT-IR spectroscopy, differential thermal analysis (DTA) and AC impedance spectroscopy. The highly conductive)gamma-phase was effectively stabilized at room temperature for compositions with x >= 0.06 whose thermal stability increases with Mg content. The complex plane plots of impedance for polycrystalline samples showed the variation of equivalent circuit models with composition at isothermal conditions. It was found that the highest value of conductivity 1.5 x 10(-3) S.cm(-1) at 300 degrees C is obtained from the composition x = 0.06 whose conductivity at 600 degrees C also reaches its maximum value of 4.9 x 10(-2) S.cm(-1). (C) 2014 Elsevier B.V. All rights reserved.