This paper describes the phase and structural relations between three polymorphs of Bi5Pb3O10.5. The low-temperature stable phase namedβLcrystallizes in the triclinic system witha=14.903(1) Å,b=14.184(1) Å,c=7.2115(7) Å,α=97.216(8)°,β=118.434(6)°,γ=80.647(8)°, andZ=4. The unquenchable high-temperature stable phase labeledβforms a solid solution and has an anti-α-AgI-type structure with the space groupIm3m;β-Bi5Pb3O10.5hasa=4.40 Å at 600°C andZ=2 (Bi5/8Pb3/8O10.5/8). The metastable phase termedβ2also forms a solid solution and crystallizes in the tetragonal system with the possible space groupP42/n;β2-Bi5Pb3O10.5hasa=12.132(1) Å,c=20.059(2) Å, andZ=9. TheβLphase transforms to theβphase at about 585°C; on subsequent cooling, at about 560°C, theβphase changes to theβ2phase which is kept to room temperature. Theβ2phase reverts to theβLphase by annealing at low temperatures, e.g., at 500°C. The structure ofβL-Bi5Pb3O10.5is based on a pseudo-bccβ-type subcell, and the transformation matrix is (–2, 2, 2)/(1, –1, 3)/(32,12, –12). The structure of theβ2phase is connected to that of red tetragonal PbO-type structure according to (3, 0, 0)/(0, 3, 0)/(0, 0, 4). From the viewpoint of the lone pair of electrons on Bi3+and Pb2+, the metastability of theβ2phase is discussed.