New compounds, Bi23M4O44.5 (M = P and V), have been found in the systems Bi2O3-M2O5. They crystallize in the triclinic system with a = 11.366(1[) Angstrom, b = 11.369(1) Angstrom, c = 20.453(3) Angstrom, alpha = 77.535(8)degrees, beta = 86.214(9)degrees, and gamma = 119.565(7)degrees for M = P, and with a = 11.545(2) Angstrom, b = 11.547(1) Angstrom, c = 20.665(3) Angstrom, alpha = 76.27(1)degrees, beta = 87.51(1)degrees, and gamma = 119.82(1)degrees for M = V. Two formula weights are contained in the unit cell. The structure is based on a pseudo-fee subcell with a’ approximate to 5.5 Angstrom. The axial relations between the triclinic cell and the subcell are a = b approximate to (3 root 2/2)a’ and c approximate to (3 root 6/2)a’. The compounds show a good oxide-ion conduction. In particular, Bi23V4O44.5 is excellent : sigma approximate to 10(-2) mho cm(-1), activation energy approximate to 0.78 eV, and O-2 transport number approximate to 0.9 at about 600 degrees C. Both compounds melt congruently at about 950 degrees C without polymorphic transformation.