This thesis describes the compositional limits, phase stability and electrical properties of several oxide ion conducting phases in the Bi₂O₃-V₂O₅ binary and Bi₂O₃-V₂O₅-Na₂O ternary phase diagrams.  Samples were prepared either by the mixed oxide route frm Bi₂O₃, V₂O₅, and Na₂CO₃ reagents or by hydrothermal processing of Bi₂O₃ and V₂O₅ using (2-3 M) NaOH.  Sample were analysed using a combination of techniques including x-ray (XRD) and electron diffraction (ED), thermal analysis (DTA/TGA), electron probe micro-probe analysis (EPMA), and impedance spectroscopy (IS). The well-known oxide ion conductor, based on the ideal composition Bi₄:V = 2:1) was shown to exist as a solid solution phase between 2.15:1 = 2.5:1 at 800 - 850°C.  XRD, ED, and EPMA data provide evidence for a new polymorph (d) for Bi-rich compositions based on a defect fluorite d-Bi₂O₃ cell, in addition to the well-known a, b, and g polymorphs.  This solid solution exhibits the highest level of oxide ion conductivity (at 800°C) of all the phases studied, with values of ~ 0.1 Scm^-1 at 800°C.  Post-annealing samples at 650°C for extended periods show that the Bi-rich solid limit is strongly temperature-dependent and hat compositions 2.4-2.5:1 are metastable and decompose into phase mixtures which include a low temperature triclinic phase based on a face-centred cubic cell with composition Bi_2.9VO_6.85. The next solid solution phase occurs at 3.5-3.75:1 and not at 4:1 as has been previously believed.  Selected area electron diffraction (SAED) revealed complex crystal chemistry with the existence of three different types of patterns all based on a 3-fold multiple of a d-Bi₂O₃ fluorite-type subcell.  Most crystals had incommensurate modulation and also streaking indicative of structural disorder, attributed to variations in the stacking order of V₄O₁₀ units within the cell. Samples were stable after post-annealing at lower temperatures, suggesting they are thermodynamically stable, albeit with complex, low crystal symmetry.  They are modest oxygen ion conductors with s-5 mScm^-1 at 800°C.