The Ba3ZnTa2O9 (BZT) and Ba3MgTa2O9 (BMT) ceramics, a family of A3B2+B5+2O9 complex perovskites, are extensively utilized in mobile based technologies due to their intrinsic high unloaded quality factor, high dielectric constant and a low (near-zero) resonant frequency temperature coefficient at microwave frequencies. The preparation conditions as well as size and nature of B cations have a profound effect on the final dielectric properties. In this article, we report the effect of Nb5+ at the Ta5+ site on the BMT structure prepared at four synthesis temperatures (1300, 1400, 1500 and 1600 °C). The analysis has been carried out using the Rietveld technique on the X-ray powder diffraction data. Results suggest that both the preparation temperatures and Nb5+ content have significant effect on the ordering of B cations in the Ba3Mg(Ta1-xNbx)2O9 solid solution. A disordered (cubic) structure is preferred by the 1300 °C compounds. The weight percentage of the ordered (trigonal) phase escalates, for a given composition, with increasing calcination temperature. A fully ordered trigonal arrangement exists only for x = 0.0 and 0.2 compounds calcined at 1600 °C, and the rest are biphasic (cubic and trigonal). The increase in the cubic fraction upon Nb5+ augmentation suggests that the solid solution leans more toward the disordered structural arrangement of B2+ and B5+ cations. © 2007 Elsevier Ltd. All rights reserved.