The structure and electrical conductivity of compositions in the pseudo-binary system Bi2MgO4-Bi2VO5.5-delta were investigated beyond the BIMEVOX stabilisation region using X-ray powder diffraction and ac impedance spectroscopy. Compositions of the general formula BiV1-xMgxO5.5-3x/2 were studied and several discrete phases were identified, viz: x=0.30, orthorhombic BIMGVOX; x=0.50, Bi8V2O17; x=0.60, Bi12V2O23; x=0.80, a delta-Bi2O3-type phase; and x=0.90, a gamma-Bi2O3-type phase. In between these compositions, mixtures of phases are observed. High-temperature diffraction reveals phase separation at x=0.30 and 0.90. The structural data correlate well with variations in activation energy and conductivity.