A complete solid-solution series, Bi23V4-4xP4xO44.5 (0 less than or equal to x less than or equal to 1), has been formed between two isomorphous compounds, Bi23M4O44.5 (M = P and V). The series crystallize in the triclinic symmetry and show a high oxide-ion conduction. The structure has been examined on single crystals of Bi23P4O44.5 using crystal lattice images taken by high-resolution transmission electron microscope. As a result, Bi and P cations are orderly located at all pseudo-cube corners and face centers of pseudo-fcc subcells with a' approximate to 0.55 nm, and the ordering of these two kinds of cations leads to the triclinic superstructure. Using this structural model, positions of oxide ions are deduced from the fluorite-related type structure.